Add .test-mojom to mojom formatted types

These files are testonly but can still be formatted by the
mojom formatter, so add to the matched extensions.

Bug: 397343888
Change-Id: I60352e8400e68426f262eb5601b76c17c0f0b912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6279546
Reviewed-by: Scott Lee <ddoman@chromium.org>
Reviewed-by: Andy Perelson <ajp@google.com>
Commit-Queue: Alex Gough <ajgo@chromium.org>

git_cl.py

@@ -7096,7 +7096,7 @@ def CMDformat(parser, args):
     if opts.python is not False:
         formatters += [(['.py'], _RunYapf)]
     if opts.mojom:
-        formatters += [(['.mojom'], _RunMojomFormat)]
+        formatters += [(['.mojom', '.test-mojom'], _RunMojomFormat)]
     if opts.lucicfg:
         formatters += [(['.star'], _RunLUCICfgFormat)]