Resolve test suite discovery import errors due to path ordering (#22454)

[Issue #22453](#22453)

- Once starting to run discovery, add the projects root path to PATH at
index 0 so that any further imports will use the projects root directory
and not reference the incorrect directory.
- Since the test suite only allows the start_dir to be one directory
deep, we can conclude that if the start_dir is not "." or contains a
"/", that we need to add that start_dir's parent to PATH. Otherwise, we
simply add the start_dir to PATH.

pythonFiles/unittestadapter/discovery.py

@@ -73,6 +73,11 @@ def discover_tests(
     }
     """
     cwd = os.path.abspath(start_dir)
+    if "/" in start_dir:  #  is a subdir
+        parent_dir = os.path.dirname(start_dir)
+        sys.path.insert(0, parent_dir)
+    else:
+        sys.path.insert(0, cwd)
     payload: PayloadDict = {"cwd": cwd, "status": "success", "tests": None}
     tests = None
     error: List[str] = []