Ignore test artifacts in .libs

Some tests may leave artifacts in the .libs directory. Ignore this
directory when detecting expected artifacts.

(cherry picked from commit cf606a9ead)

bin/tests/system/conftest.py

@@ -357,6 +357,7 @@ def logger(request, system_test_name):
 @pytest.fixture(scope="module")
 def expected_artifacts(request):
     common_artifacts = [
+        ".libs/*",  # possible build artifacts, see GL #5055
         "ns*/named.lock",
         "ns*/named.run",
         "ns*/named.run.prev",