Bug 1345109 - Ensure |mach python-test| errors out if no tests are found, r=mshal

Because test_objects was a generator, using it in the condition always returned True,
even if no tests were found. But extending test_objects to the manifest, converts it
to a list. So this patch simply moves the 'no tests' check a bit later on.

MozReview-Commit-ID: JpETWD1WQWH

python/mach_commands.py

@@ -113,15 +113,15 @@ class MachCommands(MachCommandBase):
                 # Otherwise just run everything in PYTHON_UNITTEST_MANIFESTS
                 test_objects = resolver.resolve_tests(flavor='python')
 
-        if not test_objects:
+        mp = TestManifest()
+        mp.tests.extend(test_objects)
+
+        if not mp.tests:
             message = 'TEST-UNEXPECTED-FAIL | No tests collected ' + \
                       '(Not in PYTHON_UNITTEST_MANIFESTS?)'
             self.log(logging.WARN, 'python-test', {}, message)
             return 1
 
-        mp = TestManifest()
-        mp.tests.extend(test_objects)
-
         filters = []
         if subsuite == 'default':
             filters.append(mpf.subsuite(None))