[infra] Fix polymorphic launcher testers filters.

The code generating the properties for polymorphic launchers is
generating the tester_filter field of the property incorrectly: it
should map to a TesterFilter message with a repeated field of
BuilderGroupAndName rather than just a repeated field of
BuilderGroupAndName.

Bug: 1346396
Change-Id: Iaca79a4edd2bf8971234fcf12a480b5dab8f443f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4097368
Commit-Queue: Kuan Huang <kuanhuang@chromium.org>
Reviewed-by: Kuan Huang <kuanhuang@chromium.org>
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1081931}