tests: Fix distcheck.

* tests/Makefile.am (SUBDIRS): Add 'pkits' again.  Simply dropping it
makes 'make distcheck' unhappy.
* tests/pkits/Makefile.am (TESTS): Remove all tests.

Disable the 'pkits' tests in a way that keeps 'make distcheck' happy.
To run tests individually, do

make -Ctests/pkits check TESTS=some-test

• GnuPG-bug-id: T3067
• Fixes-commit: af1c1a57e46a00a32d83c1a58c5f3ef6f4a1c1d1
• Signed-off-by: Justus Winter <justus@g10code.com>