improve coverage of unit tests

[prjemian]

Improve the code coverage of the unit tests. These modules have coverage below 80%

• punx.cache_manager
• punx.validations.attribute
• punx.validations.hdf5_group_items_in_base_class

[prjemian]

(bluesky_2022_1) prjemian@zap:~/.../prjemian/punx$ git grep application_definition
punx/tests/test_validate.py:def TODO_test_application_definition(hfile):
punx/tests/test_validate.py:def TODO_test_contributed_application_definition(hfile):
punx/validate.py:                    self.validate_application_definition(v_item.parent)
punx/validate.py:    def validate_application_definition(self, v_item):
punx/validate.py:        from .validations import application_definition
punx/validate.py:        application_definition.verify(self, v_item)

[prjemian]

LGTM reports that some imports of FileNotFound are unused. Actually, this is superseded by FileNotFoundError now.