releasetools: Clean up test_sign_target_files_apks.py.

Since we have been carrying test certificates in testdata/ for other
tests, do the same for test_sign_target_files_apks.py. Copy
verity.x509.pem from build/target/product/security/ to testdata/ for
that purpose.

Also capture the stderr output in ReplaceVerityKeyId().

Test: python -m unittest test_sign_target_files_apks
Change-Id: Ie11e042086952e8a4a5a63950cb0b16cc436b7e6