QA: Add default setting for tests

Commit 1010ec70 enabled all tests by default. In some scenarios where
the configuration is static one may not want newly added tests to be
run.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
(cherry picked from commit 59a8fe48)

qa/qa-sample.json

@@ -51,6 +51,9 @@
   },
 
   "tests": {
+    "# Whether tests are enabled or disabled by default": null,
+    "default": true,
+
     "env": true,
     "os": true,
     "tags": true,

qa/qa_config.py

@@ -68,7 +68,14 @@ def TestEnabled(tests):
   """
   if isinstance(tests, basestring):
     tests = [tests]
-  return compat.all(cfg.get("tests", {}).get(t, True) for t in tests)
+
+  # Get settings for all tests
+  all_tests = cfg.get("tests", {})
+
+  # Get default setting
+  default = all_tests.get("default", True)
+
+  return compat.all(all_tests.get(name, default) for name in tests)
 
 
 def GetMasterNode():