[SVLS-4647] Fix unittests

[joeyzhao2018]

What does this PR do?

Due to changes made in dd-trace-py, the data about _meta={'_dd.p.dm': '-3'} is different from our hardcoded mocks.
So instead of hardcoding them, we might as well just directly use the propagator to get the expected items.

• also refactored repeated code into a common_tests_with_trace_context_extraction_injection method.

The Files Changed tab is messed up ...

Due to the refactoring, the files changed is pretty hard to read. To help the reviewers, the change is essentially changing

        lambda_ctx = get_mock_context()
        ctx, source, event_source = extract_dd_trace_context(
            {
                "headers": {
                    TraceHeader.TRACE_ID: "123",
                    TraceHeader.PARENT_ID: "321",
                    TraceHeader.SAMPLING_PRIORITY: "1",
                }
            },
            lambda_ctx,
        )
        self.assertEqual(source, "event")
        expected_context = Context(trace_id=123, span_id=321, sampling_priority=1)
        self.assertEqual(ctx, expected_context)
        self.assertDictEqual(
            get_dd_trace_context(),
            {
                TraceHeader.TRACE_ID: "123",
                TraceHeader.PARENT_ID: fake_xray_header_value_parent_decimal,
                TraceHeader.SAMPLING_PRIORITY: "1",
            },
        )
        create_dd_dummy_metadata_subsegment(ctx, XraySubsegment.TRACE_KEY)
        self.mock_send_segment.assert_called()
        self.mock_send_segment.assert_called_with(
            XraySubsegment.TRACE_KEY,
            expected_context,
        )

into

        headers = {
            TraceHeader.TRACE_ID: "123",
            TraceHeader.PARENT_ID: "321",
            TraceHeader.SAMPLING_PRIORITY: "1",
        }
        ctx, source, event_source = extract_dd_trace_context(
            event_containing_headers,
            lambda_context,
        )
        self.assertEqual(source, "event")
        expected_context = propagator.extract(headers)
        self.assertEqual(ctx, expected_context)
        create_dd_dummy_metadata_subsegment(ctx, XraySubsegment.TRACE_KEY)
        self.mock_send_segment.assert_called()
        self.mock_send_segment.assert_called_with(
            XraySubsegment.TRACE_KEY,
            expected_context,
        )
        # when no active ddtrace context, xray context would be used
        expected_context.span_id = int(fake_xray_header_value_parent_decimal)
        expected_headers = {}
        propagator.inject(expected_context, expected_headers)
        dd_context_headers = get_dd_trace_context()
        self.assertDictEqual(expected_headers, dd_context_headers)

Motivation

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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• This PR passes the integration tests (ask a Datadog member to run the tests)

[duncanista]

LGTM, -172 lines, nice!