TestService: Add cros_signing tests endpoint.

BUG=chromium:992927
TEST=run_tests, manually ran endpoint

Change-Id: I22bd6a81113c973809330d7cdc0e7e5899ce6eb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1757097
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/test.py b/api/controller/test.py
index b3d5c90..51ac394 100644
--- a/api/controller/test.py
+++ b/api/controller/test.py
@@ -174,3 +174,13 @@
     cache_dir = test.PrepareMoblabVmImageCache(vms, builder, cache_payload_dirs)
     test.RunMoblabVmTest(chroot, vms, builder, cache_dir, results_dir)
     test.ValidateMoblabVmTest(results_dir)
+
+
+@validate.validation_complete
+def CrosSigningTest(_input_proto, _output_proto, _config):
+  """Run the cros-signing unit tests."""
+  test_runner = os.path.join(constants.SOURCE_ROOT, 'cros-signing', 'signer',
+                             'run_tests.py')
+  result = cros_build_lib.RunCommand([test_runner], error_code_ok=True)
+
+  return result.returncode