Add tests for the branches the first mutation run left alive.

tests/conftest.py

@@ -0,0 +1,12 @@
+"""Keep a mutant from launching coverage against the project under test.
+
+`run()` falls through into `run_coverage` when a mutant skips the help return.
+That would delete `target/coverage` and recurse into this suite.
+"""
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def do_not_launch_project_coverage(monkeypatch):
+    monkeypatch.setattr("crapper.cli.run_coverage", lambda *_args, **_kwargs: None)