Add test for soong/testing/test_spec.

This CL adds test for test_spec.go and all_test_specs.go (singleton).

Bug: 296873595
Test: m nothing --no-skip-soong-tests -j96

Ignore-AOSP-First: CPing test_spec rule to udc-mainline-prod to support migration of test targets. Cherry pick of:aosp/2836072


Change-Id: I5010c68512e75d1b9a337c02da86faac15e376fe
Merged-In: I5010c68512e75d1b9a337c02da86faac15e376fe