tests: Reduce iterations / threads

* tests/gpg/t-gpgconf.c (main): Reduce iterations to 10.
* tests/gpg/t-thread-keylist-verify.c,
tests/gpg/t-thread-keylist.c (THREAD_COUNT): Reduce to 10.

While these tests tested for race conditions a smaller number
of iteration should still show problems if they are run on
multiple systems and regulary. While the 100 Thread count in
the t-thread tests could lead to resource problems.