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/conftest.py

@@ -0,0 +1,59 @@
+"""Configurations and utilities for all tests."""
+
+import pytest
+
+
+def pytest_addoption(parser):
+    """Define custom CLI options for `pytest`."""
+    parser.addoption(
+        "--smoke-tests-only",
+        action="store_true",
+        default=False,
+        help="Run the minimum number of (unit) tests to achieve full coverage",
+    )
+
+
+def pytest_configure(config):
+    """Define custom markers explicitly."""
+    config.addinivalue_line(
+        "markers",
+        "integration_test: non-unit test case; skipped during coverage reporting",
+    )
+    config.addinivalue_line(
+        "markers",
+        "overlapping_test: test case not contributing towards higher coverage",
+    )
+    config.addinivalue_line(
+        "markers",
+        "sanity_test: test case providing confidence in the test data",
+    )
+
+
+def pytest_collection_modifyitems(config, items):
+    """Pre-process the test cases programatically.
+
+    - Add `no_cover` marker to test cases with any of these markers:
+      + "integration_test"
+      + "overlapping_test"
+      + "sanity_test"
+    - Select test cases with none of the above markers as smoke tests,
+      i.e., the minimum number of test cases to achieve 100% coverage
+    """
+    smoke_tests = []
+
+    for item in items:
+        if (
+            "integration_test" in item.keywords
+            or "overlapping_test" in item.keywords
+            or "sanity_test" in item.keywords
+        ):
+            item.add_marker(pytest.mark.no_cover)
+
+        elif config.getoption("--smoke-tests-only"):
+            smoke_tests.append(item)
+
+    if config.getoption("--smoke-tests-only"):
+        if not smoke_tests:
+            pytest.exit("No smoke tests found")
+
+        items[:] = smoke_tests