Treat methodName="runTest" similar to unittest.TestCase

pytest 8.2 relies on a feature of `unittest.TestCase` where the
initialization of it with the default `methodName="runTest"` is treated
specially, allowing it to insantiate even without `runTest` method
actually existing.

See under "Changed in Python 3.2" in unittest.TestCase docs

This fixes the error with pytest 8.2:
  AttributeError: 'TestUtil' object has no attribute 'runTest'. Did you mean: 'subTest'?

Fixes: testing-cabal#372
ref: https://docs.python.org/3/library/unittest.html#unittest.TestCase
ref: https://github.com/python/cpython/blob/51aefc5bf907ddffaaf083ded0de773adcdf08c8/Lib/unittest/case.py#L419-L426
ref: pytest-dev/pytest#12263 (comment)

testtools/testcase.py

@@ -735,7 +735,17 @@ def _run_teardown(self, result):
 
  def _get_test_method(self):
  method_name = getattr(self, "_testMethodName")
- return getattr(self, method_name)
+ try:
+ m = getattr(self, method_name)
+ except AttributeError:
+ if method_name != "runTest":
+ # We allow instantiation with no explicit method name
+ # but not an *incorrect* or missing method name.
+ raise ValueError(
+ "no such test method in %s: %s" % (self.__class__, method_name)
+ )
+ else:
+ return m
 
  def _run_test_method(self, result):
  """Run the test method for this test.