Create a test case for testing L2 configuration. (sonic-net#14714)

Reference: https://github.com/sonic-net/SONiC/wiki/L2-Switch-mode

### Description of PR
Create a test case for configuring switch to L2 mode.

Summary:
Add a test case for writing L2 configuration into config DB. This is precursor to a testcase requested in
issue 8777 sonic-net#8777

### Approach
Add a test case
#### What is the motivation for this PR?
Address sonic-net#8777

#### How did you do it?

#### How did you verify/test it?
Ran on virtual switch.

tests/l2/test_l2_configure.py

@@ -0,0 +1,161 @@
+"""
+Tests related to L2 configuration
+"""
+
+import logging
+import pytest
+
+from tests.common import config_reload
+from tests.common.platform.processes_utils import wait_critical_processes
+from tests.common.helpers.assertions import pytest_assert
+
+CONFIG_DB = "/etc/sonic/config_db.json"
+CONFIG_DB_BAK = "/etc/sonic/config_db.json.bak"
+DUT_IMG_PATH = "/tmp/dut-sonic-img.bin"
+LOCALHOST_IMG_PATH = "/tmp/localhost-sonic-img.bin"
+
+logger = logging.getLogger(__name__)
+
+pytestmark = [
+    pytest.mark.topology("t0"),
+    pytest.mark.sanity_check(skip_sanity=True),
+    pytest.mark.disable_loganalyzer,
+    pytest.mark.skip_check_dut_health,
+]
+
+
+@pytest.fixture(autouse=True)
+def setup_env(duthosts, rand_one_dut_hostname):
+    """
+    Setup/teardown fixture for each loopback interface test.
+    rollback to check if it goes back to starting config
+
+    Args:
+        duthosts: list of DUTs.
+        rand_selected_dut: The fixture returns a randomly selected DuT.
+    """
+    duthost = duthosts[rand_one_dut_hostname]
+    duthost.shell("sudo cp {} {}".format(CONFIG_DB, CONFIG_DB_BAK))
+
+    yield
+
+    duthost.shell("sudo cp {} {}".format(CONFIG_DB_BAK, CONFIG_DB))
+    duthost.shell("sudo rm -f {}".format(CONFIG_DB_BAK))
+    config_reload(duthost)
+    wait_critical_processes(duthost)
+
+
+def is_table_empty(duthost, table):
+    """
+    @summary: Verify a table is empty.
+
+    Args:
+        duthost: DUT host object.
+        table: Table name to verify.
+    """
+    # grep returns 1 when there is no match, use || true to override that.
+    count = int(
+        duthost.shell(
+            'sonic-db-cli CONFIG_DB KEYS "{}|*" | grep -c {} || true'.format(
+                table, table
+            )
+        )["stdout"]
+    )
+    return count == 0
+
+
+def get_db_version(duthost):
+    """
+    @summary: Get the current database version.
+
+    Args:
+        duthost: DUT host object.
+    """
+    try:
+        return duthost.shell('sonic-db-cli CONFIG_DB HGET "VERSIONS|DATABASE" VERSION')[
+            "stdout"
+        ]
+    except Exception as e:
+        logger.error("Failed to get database version: {}".format(e))
+        return ""
+
+
+def test_no_hardcoded_minigraph(duthosts, rand_one_dut_hostname, tbinfo):
+    """
+    @summary: A testcase asserts no hardcoded minigraph config is imported to config_db during L2 configuration.
+
+    Args:
+        duthosts: list of DUTs.
+        rand_one_dut_hostname: The fixture returns a randomly selected DuT.
+        tbinfo: The testbed information. Needed for configuring management interface.
+
+    """
+    # Setup.
+    duthost = duthosts[rand_one_dut_hostname]
+    if is_table_empty(duthost, "TELEMETRY") or is_table_empty(duthost, "RESTAPI"):
+        pytest.skip("TELEMETRY or RESTAPI table is empty. Please load minigraph first.")
+
+    hwsku = duthost.facts["hwsku"]
+    mgmt_fact = duthost.get_extended_minigraph_facts(tbinfo)["minigraph_mgmt_interface"]
+
+    # Step 2: Configure DUT into L2 mode.
+    # Save original config
+    duthost.shell("sudo cp {} {}".format(CONFIG_DB, CONFIG_DB_BAK))
+    # Perform L2 configuration
+    L2_INIT_CFG_FILE = "/tmp/init_l2_cfg.json"
+    MGMT_CFG_FILE = "/tmp/mgmt_cfg.json"
+    L2_CFG_FILE = "/tmp/l2_cfg.json"
+    gen_l2_cfg = "sudo sonic-cfggen --preset l2 -p -H -k {} > {}".format(
+        hwsku, L2_INIT_CFG_FILE
+    )
+    duthost.shell(gen_l2_cfg)
+    gen_mgmt_cfg = """
+    echo '
+    {{
+        "MGMT_INTERFACE": {{
+            "eth0|{}/{}": {{
+                "gwaddr": "{}"
+            }}
+        }},
+        "DEVICE_METADATA": {{
+            "localhost": {{
+                "hostname": "{}"
+            }}
+        }},
+        "MGMT_PORT": {{
+            "eth0": {{
+                "admin_status": "up",
+                "alias": "eth0"
+            }}
+        }}
+    }}' > {}
+    """.format(
+        mgmt_fact["addr"],
+        mgmt_fact["prefixlen"],
+        mgmt_fact["gwaddr"],
+        duthost.hostname,
+        MGMT_CFG_FILE,
+    )
+    duthost.shell(gen_mgmt_cfg)
+    duthost.shell(
+        "jq -s '.[0] * .[1]' {} {} > {}".format(
+            L2_INIT_CFG_FILE, MGMT_CFG_FILE, L2_CFG_FILE
+        )
+    )
+    duthost.shell("sudo cp {} {}".format(L2_CFG_FILE, CONFIG_DB))
+    db_version_before = get_db_version(duthost)
+    logger.info(
+        "Database version before L2 configuration reload: {}".format(db_version_before)
+    )
+    config_reload(duthost)
+    wait_critical_processes(duthost)
+    db_version_after = get_db_version(duthost)
+    logger.info(
+        "Database version after L2 configuration reload: {}".format(db_version_after)
+    )
+
+    # Verify no minigraph config is present.
+    for table in ["TELEMETRY", "RESTAPI"]:
+        pytest_assert(
+            is_table_empty(duthost, table), "{} table is not empty!".format(table)
+        )