fix(test): tighten container-test detection in run-unit-tests

The container-vs-fast heuristic used plain `'podman' in content` and
`'testcontainers' in content` substring matches, which also fired on
comments, docstrings, and on plugin names that happen to contain the
word `podman`. As a result, `apache-httpd-status`, `deb-updates`,
`needs-restarting`, `rpm-updates`, `podman-info`, and `podman-stats`
were silently skipped by `tools/run-unit-tests --no-container` and
therefore by the full `tox` matrix.

Match on precise signatures instead: `import testcontainers`,
`from testcontainers`, `lib.lftest.run_container(`, `DockerImage(`,
and `DockerContainer(`. Those strings only appear in real container
drivers, not in incidental mentions of the word.

Author: markuslf

tools/run-unit-tests

@@ -47,22 +47,36 @@ import sys
 SKIP_PLUGINS = {'example'}
 
 
+# Signatures that identify a real container-driven unit test. These must
+# be precise substring matches: a plain keyword like `podman` would also
+# fire on comments, docstrings, or on plugins whose name happens to
+# contain `podman` (e.g. `podman-info`, `podman-stats`), which used to
+# cause fixture-based tests to be silently skipped by the multi-Python
+# matrix.
+CONTAINER_TEST_SIGNATURES = (
+    'import testcontainers',
+    'from testcontainers',
+    'lib.lftest.run_container(',
+    'DockerContainer(',
+    'DockerImage(',
+)
+
+
 def is_container_test(check_plugins, plugin):
     """Return True if the plugin's unit test actually drives a container runtime.
 
     We look at the `run` file's content rather than the presence of a
     `containerfiles/` subdirectory: a plugin may keep containerfiles around
     for a future testcontainers migration while its `run` file is already
-    fixture-based and fast. The run file counts as a container test when
-    it references `podman` as a subprocess target or imports `testcontainers`.
+    fixture-based and fast.
     """
     run_file = os.path.join(check_plugins, plugin, 'unit-test', 'run')
     try:
         with open(run_file, encoding='utf-8') as f:
             content = f.read()
     except OSError:
         return False
-    return 'podman' in content or 'testcontainers' in content
+    return any(sig in content for sig in CONTAINER_TEST_SIGNATURES)
 
 
 def main():