Clean up pytest warnings

[GregoryComer]

When running pytest, there are a few classes that pytest thinks are test classes, but actually aren't. Most culprits are test-related types with Test in the name but not containing actual test methods. This leads to several pages of of warnings like the following:

# pytest

Users/gjcomer/src/executorch/backends/test/suite/reporting.py:152: PytestCollectionWarning: cannot collect test class 'TestCaseSummary' because it has a __init__ constructor (from: backends/test/suite/tests/test_reporting.py)
  @dataclass
/Users/gjcomer/src/executorch/backends/test/suite/reporting.py:60: PytestCollectionWarning: cannot collect test class 'TestResult' because it has a __init__ constructor (from: backends/test/suite/tests/test_reporting.py)
  class TestResult(IntEnum):
/Users/gjcomer/src/executorch/backends/test/suite/reporting.py:211: PytestCollectionWarning: cannot collect test class 'TestSessionState' because it has a __init__ constructor (from: backends/test/suite/tests/test_reporting.py)
  @dataclass
/Users/gjcomer/src/executorch/src/executorch/backends/xnnpack/test/tester/tester.py:104: PytestCollectionWarning: cannot collect test class 'Tester' because it has a __init__ constructor (from: backends/xnnpack/test/fragments/test_dim_order.py)
  class Tester(TesterBase):
/Users/gjcomer/src/executorch/src/executorch/backends/xnnpack/test/tester/tester.py:104: PytestCollectionWarning: cannot collect test class 'Tester' because it has a __init__ constructor (from: backends/xnnpack/test/ops/test_abs.py)
  class Tester(TesterBase):
...

To solve this, we can do the following:

• Set up a conda or other Python environment (docs).
• Run install_executorch.sh --editable from the root of the executorch source directory.
• Run pytest from the root of the executorch source directory.
• For each non-test class generating a PytestCollectionWarning, add __test__ = False (see https://stackoverflow.com/questions/58624641/how-to-prevent-pytestcollectionwarning-when-testing-class-testament-via-pytest).

To validate:

• Run pytest again. We shouldn't see any PytestCollectionWarnings with relation to classes not containing tests.