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.