more inference tests (#1908)

## Changes
more tests

### Linked issues
Progresses #1901 

### Functionality 
None

### Tests
- [x] added unit tests

Co-authored-by: Eric Vergnaud <[email protected]>

tests/unit/source_code/linters/test_python_ast.py

@@ -137,6 +137,20 @@ def test_infers_fstring_value():
     assert strings == ["Hello abc!"]
 
 
+def test_infers_string_format_value():
+    source = """
+value = "abc"
+fstring = "Hello {0}!".format(value)
+"""
+    tree = Tree.parse(source)
+    nodes = tree.locate(Assign, [])
+    tree = Tree(nodes[1].value)  # value of fstring = ...
+    values = list(tree.infer_values())
+    assert all(value.is_inferred() for value in values)
+    strings = list(value.as_string() for value in values)
+    assert strings == ["Hello abc!"]
+
+
 def test_infers_fstring_values():
     source = """
 values_1 = ["abc", "def"]