Adjust check for .stestr directory

stestr can leave a .stestr directory from which subunit cannot be
generated after an initial run that fails due to a python import/parse
error.

In order to determine if a run will be able to generate subunit output,
check for the '.stestr/failing' file.

NOTE: This is **NOT** an indication of a failing run. It's a file that
will be present in the .stestr directory if an entire run has happened
and acts as a marker that the directory has enough information to export
a subunit stream.

Change-Id: Id1dd696c81da09e9d2e5fe350337b73eb0646d2d

roles/fetch-subunit-output/tasks/main.yaml

@@ -1,6 +1,10 @@
+# NOTE(mordred) Check for the failing file in the .stestr directory instead of
+# just the directory. An stestr run that fails due to python parsing errors
+# will leave a directory but with no test results, which will result in an
+# error in the subunit generation phase.
 - name: Check for stestr directory
   stat:
-    path: "{{ zuul_work_dir }}/.stestr"
+    path: "{{ zuul_work_dir }}/.stestr/failing"
   register: stestr_stat
 
 - name: Generate stestr subunit file