Fix a couple of issues with the test_structmembersType class in _testcapimodule

Fix a couple of issues with the test_structmembersType class in _testcapimodule

 - rename to _test_structmembersType to avoid the class being automatically
   called by test_capi

 - allow space for trailing NUL in inplace_member field of all_structmembers

 - use T_STRING_INPLACE instead of T_INPLACE_STRING as keyword argument
   to _test_structmembersType initializer

 - don't attempt to initialize inplace_member field if T_STRING_INPLACE
   argument wasn't supplied.