cft: re-enable label flag for cros-test

BUG=b:228645277
TEST=unittest

Cq-Depend: chromium:3611642
Change-Id: I979a479aeb65f139b0aaff1396ebf44520212278
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3610695
Reviewed-by: Jaques Clapauch <jaquesc@google.com>
Reviewed-by: Azizur Rahman <azrahman@google.com>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Tested-by: Derek Beckett <dbeckett@chromium.org>
diff --git a/api/controller/test.py b/api/controller/test.py
index 1c7a932..ec1ef7c 100644
--- a/api/controller/test.py
+++ b/api/controller/test.py
@@ -271,10 +271,15 @@
       cmd += ['--tags', tags]
       cmd += ['--output', output_path]
 
-      # Labels aren't support yet in cloudbuild script.
+      # Label flag is different for cros-test.
       if human_name != 'cros-test':
         cmd += labels
 
+      if human_name == 'cros-test' and labels:
+        # Translate generator to comma separated string.
+        ct_labels = ','.join(labels)
+        cmd += ['--labels', ct_labels]
+
       result = test_pb2.TestServiceContainerBuildResult()
       result.name = human_name