Add test-only for test-per-src cc modules

When testing test-only validations, I noticed that some cc_test modules
where not reporting that they were test-only.

This happened because test-per-src variations were short-circuiting
writing out the provider.

The `all_teams` target is using the first variation for a module, but
it possibly should see if any variation is test-only.
For now, just making this fix to cc.

Test: m all_teams  && gqui from  "flatten(out/soong/ownership/all_teams.pb, teams)" proto build/soong/android/team_proto/team.proto:AllTeams ' where teams.kind = "cc_test" and teams.target_name="libnativebridge-tests"'
Test: go test ./cc
Change-Id: I6d44a521f5f2457527049399509d979559d7dc17