core: Prevent wrong plaintext when verifying clearsigned signature

* src/engine-gpg.c (gpg_verify): Use a separate pipe instead of stdout
for reading the plaintext.
* tests/gpg/t-support.h (PGM): Define if undefined.
(print_data): Undefine BUF_SIZE.
(check_data): New.
* tests/gpg/t-verify.c (clearsigned_plus_key_block): New.
(main): Add test.

Reading the plaintext from stdout is a bad idea because gpg can also
print other stuff on stdout, e.g. the keys contained in a public key
block. This is fixed by reading the plaintext via a special pipe.

• GnuPG-bug-id: T6622