tests: Increase test coverage

- Add a `temp_workspace` context manager to simplify writing new tests.
- Add `# pragma: no cover` to unit test code paths used for skipping tests.
  These code paths are only covered when tests are skipped.
  That makes it impractical to reach full code coverage on the unit test code.
  Having full coverage of unit tests is helpful for identifying unused tests.
- Test the `octal-values` rule with a custom tag.
- Test the cli `-d` option with the `default` config.
- Test support for the `XDG_CONFIG_HOME` env var.
- Test warning message output.
- Test support for `.yamllint.yml` config files.
- Test support for `.yamllint.yaml` config files.
- Test error handling of a rule with a non-enable|disable|dict value.
- Test error handling of `ignore` with a non-pattern value.
- Test error handling of a rule `ignore` with a non-pattern value.
- Test error handling of `locale` with a non-string value.
- Test error handling of `yaml-files` with a non-list value.
- Test extending config containing `ignore`.
- Test `LintProblem.__repr__` without a rule.
- Test `LintProblem.__repr__` with a rule.

tests/rules/test_key_ordering.py

@@ -116,7 +116,7 @@ class KeyOrderingTestCase(RuleTestCase):
         self.addCleanup(locale.setlocale, locale.LC_ALL, (None, None))
         try:
             locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
-        except locale.Error:
+        except locale.Error:  # pragma: no cover
             self.skipTest('locale en_US.UTF-8 not available')
         conf = ('key-ordering: enable')
         self.check('---\n'
@@ -135,7 +135,7 @@ class KeyOrderingTestCase(RuleTestCase):
         self.addCleanup(locale.setlocale, locale.LC_ALL, (None, None))
         try:
             locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
-        except locale.Error:
+        except locale.Error:  # pragma: no cover
             self.skipTest('locale en_US.UTF-8 not available')
         conf = ('key-ordering: enable')
         self.check('---\n'

tests/rules/test_octal_values.py

@@ -32,6 +32,7 @@ class OctalValuesTestCase(RuleTestCase):
                 '  forbid-explicit-octal: false\n'
                 'new-line-at-end-of-file: disable\n'
                 'document-start: disable\n')
+        self.check('after-tag: !custom_tag 010', conf)
         self.check('user-city: 010', conf, problem=(1, 15))
         self.check('user-city: abc', conf)
         self.check('user-city: 010,0571', conf)