pyln: Allow the base test directory to contain some files

We were not removing the base test directory if we had other files in there,
which was the case for postgres runs. This now explicitly check for `test_*`
directories which are an indicator of a failed test.

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -26,8 +26,16 @@ def test_base_dir():
 
     yield directory
 
-    if os.listdir(directory) == []:
+    # Now check if any test directory is left because the corresponding test
+    # failed. If there are no such tests we can clean up the root test
+    # directory.
+    contents = [d for d in os.listdir(directory) if os.path.isdir(d) and d.startswith('test_')]
+    if contents == []:
         shutil.rmtree(directory)
+    else:
+        print("Leaving base_dir {} intact, it still has test sub-directories with failure details: {}".format(
+            directory, contents
+        ))
 
 
 @pytest.fixture(autouse=True)