tests/gpg-verify-data: Split out signature data

The full block with all 5 signatures remains, but this allows passing
individual signatures through the GPG verification APIs. The split was
done with `gpgsplit`, and looking at the output of `gpg --list-packets`
of the split and unsplit files appears correct.

tests/gpg-verify-data/README.md

@@ -1,5 +1,7 @@
 This is a GPG config directory for use with the OstreeGpgVerifyResult
-test cases. The test data (`lgplv2`) is signed with a variety of valid
-and invalid GPG keys in a detached signature file (`lgplv2.sig`).
+test cases. The test data (`lgpl2`) is signed with a variety of valid
+and invalid GPG keys in a detached signature file (`lgpl2.sig`). In
+addition, each detached signature is available in a separate file
+(`lgpgl2.sig<N>`).
 
 The passphrase for all the keys is `redhat`.