Add support for docstring of test classes

[paxcodes]

Problem pytest-spec currently doesn't have an option to change how test classes are printed as a specification. There is a spec_header_format but that is different.

Given

class Test_class_XYZ:
    """docstring summary of Test_class_XYZ"""

    def test_case(self, result):
        """docstring summary of test_case"""

Desired

docstring summary of Test_class_XYZ:
  ✓ docstring summary of test_case     

Actual

Class XYZ:
  ✓ docstring summary of test_case

[felipecrp]

It would be a super useful feature to emulate describe and context as classes.

[felipecrp]

@pchomik are you considering implementing this feature?