tests: Prevent rare failure of gcry_pk_decrypt test.

* tests/basic.c (check_pubkey_crypt): Add special mode 1.
(main): Add option --loop.

This failure has been reported by Jussi Kivilinna. The new loop
option was needed to track that down. It took me up to 100 iterations
to trigger the bug. With the fix applied I am currently at 1000
iteration with no problems. Command line to evoke the problem was:

./basic --pubkey --verbose --loop -1 --die

• Signed-off-by: Werner Koch <wk@gnupg.org>