Add smoke tests

- extend `pytest` with an option to run only the minimum number
  of (unit) test cases to just keep the coverage at 100%
- rationale:
  + many of the unit test cases partly overlap with
    respect to the lines of source code executed
  + also, integration tests, by definition, do not
    contribute to a higher test coverage
- implementation: mark "redundant" test cases as one of:
  + `pytest.mark.integration_test`
    => code usage from the perspective of the end user
  + `pytest.mark.overlapping_test`
    => tests not contributing to the 100% coverage
  + `pytest.mark.sanity_test`
    => tests providing confidence in the test data
- add `tests.conftest` module
  => programatically convert the above markers into
     `@pytest.mark.no_cover` and collect the non-"redundant" tests
- add nox session "test-fast" to run only the minimum
  number of (unit) test while holding coverage at 100%
- refactor some test modules
  + wrap some test cases in a class
  + move sanity tests to the end of the files

tests/test_version.py

@@ -236,36 +236,7 @@ INVALID_NOT_SEMANTIC = (
 INVALID_VERSIONS = INVALID_NOT_READABLE + INVALID_NOT_SEMANTIC
 
 
-@pytest.mark.parametrize(
-    ["version1", "version2"],
-    zip(  # loop over pairs of neighboring elements
-        VALID_AND_NORMALIZED_VERSIONS,
-        VALID_AND_NORMALIZED_VERSIONS[1:],
-    ),
-)
-def test_versions_are_strictly_ordered(version1, version2):
-    """`VALID_AND_NORMALIZED_VERSIONS` are ordered."""
-    version1_parsed = pkg_version.Version(version1)
-    version2_parsed = pkg_version.Version(version2)
-
-    assert version1_parsed < version2_parsed
-
-
-@pytest.mark.parametrize(
-    ["version1", "version2"],
-    zip(  # loop over pairs of neighboring elements
-        VALID_AND_NOT_NORMALIZED_VERSIONS,
-        VALID_AND_NOT_NORMALIZED_VERSIONS[1:],
-    ),
-)
-def test_versions_are_weakly_ordered(version1, version2):
-    """`VALID_AND_NOT_NORMALIZED_VERSIONS` are ordered."""
-    version1_parsed = pkg_version.Version(version1)
-    version2_parsed = pkg_version.Version(version2)
-
-    assert version1_parsed <= version2_parsed
-
-
+@pytest.mark.overlapping_test
 class VersionClassification:
     """Classifying version identifiers.
 
@@ -344,6 +315,7 @@ class VersionClassification:
         return is_so_by_parts
 
 
+@pytest.mark.overlapping_test
 class TestVersionIdentifier(VersionClassification):
     """The versions must comply with PEP440 ...
 
@@ -504,6 +476,7 @@ class TestVersionIdentifier(VersionClassification):
         assert parsed_version.public != unparsed_version
 
 
+@pytest.mark.overlapping_test
 class TestVersionIdentifierWithPattern:
     """Test the versioning with a custom `regex` pattern."""
 
@@ -585,3 +558,36 @@ class TestUnavailablePackageMetadata:
         with self.hide_metadata_from_package("lalib") as lalib_pkg:
             assert lalib_pkg.__pkg_name__ == "unknown"
             assert lalib_pkg.__version__ == "unknown"
+
+
+@pytest.mark.sanity_test
+class TestSampleVersionData:
+    """Ensure the `VALID_*_VERSIONS` are in order."""
+
+    @pytest.mark.parametrize(
+        ["version1", "version2"],
+        zip(  # loop over pairs of neighboring elements
+            VALID_AND_NORMALIZED_VERSIONS,
+            VALID_AND_NORMALIZED_VERSIONS[1:],
+        ),
+    )
+    def test_versions_are_strictly_ordered(self, version1, version2):
+        """`VALID_AND_NORMALIZED_VERSIONS` are ordered."""
+        version1_parsed = pkg_version.Version(version1)
+        version2_parsed = pkg_version.Version(version2)
+
+        assert version1_parsed < version2_parsed
+
+    @pytest.mark.parametrize(
+        ["version1", "version2"],
+        zip(  # loop over pairs of neighboring elements
+            VALID_AND_NOT_NORMALIZED_VERSIONS,
+            VALID_AND_NOT_NORMALIZED_VERSIONS[1:],
+        ),
+    )
+    def test_versions_are_weakly_ordered(self, version1, version2):
+        """`VALID_AND_NOT_NORMALIZED_VERSIONS` are ordered."""
+        version1_parsed = pkg_version.Version(version1)
+        version2_parsed = pkg_version.Version(version2)
+
+        assert version1_parsed <= version2_parsed