tests: Disable 'pkits' test suite.

* tests/Makefile.am (SUBDIRS): Drop 'pkits'.
* tests/pkits/common.sh: Fix locating 'PKITS_data.tar.bz2'.
* tests/pkits/inittests: Likewise.

These tests are unmaintained and broken, and were previously only run
when doing in-tree builds, hence nobody noticed that they were badly
broken.

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