Fix unittestadapter test warnings by using different import names for classes that begin with Test, so that pytest doesn't try use them as tests.

[garyvdm]

When I run the python tests, I get these warnings: This PR fixes these.

========================================================= warnings summary =========================================================
python_files/unittestadapter/pvsc_utils.py:26
  /home/gary/dev/vscode-python/python_files/unittestadapter/pvsc_utils.py:26: PytestCollectionWarning: cannot collect test class 'TestNodeTypeEnum' because it has a __new__ constructor (from: tests/unittestadapter/test_discovery.py)
    class TestNodeTypeEnum(str, enum.Enum):

python_files/unittestadapter/pvsc_utils.py:45
  /home/gary/dev/vscode-python/python_files/unittestadapter/pvsc_utils.py:45: PytestCollectionWarning: cannot collect test class 'TestNode' because it has a __init__ constructor (from: tests/unittestadapter/test_utils.py)
    class TestNode(TestData):

python_files/unittestadapter/pvsc_utils.py:26
  /home/gary/dev/vscode-python/python_files/unittestadapter/pvsc_utils.py:26: PytestCollectionWarning: cannot collect test class 'TestNodeTypeEnum' because it has a __new__ constructor (from: tests/unittestadapter/test_utils.py)
    class TestNodeTypeEnum(str, enum.Enum):