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
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 1f9a3ca..c96e616 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -597,7 +597,7 @@
 
     # Extract keyid using openssl command.
     p = common.Run(["openssl", "x509", "-in", key_path, "-text"],
-                   stdout=subprocess.PIPE)
+                   stdout=subprocess.PIPE, stderr=subprocess.PIPE)
     keyid, stderr = p.communicate()
     assert p.returncode == 0, "Failed to dump certificate: {}".format(stderr)
     keyid = re.search(