Clean up FSFS pack tests.  Use a centralized check to skip these
tests for non-FSFS repositories.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (create_packed_filesystem): Skip tests for non-FSFS repositories.
  (pack_filesystem,
   pack_even_filesystem,
   recover_fully_packed): Remove the test-local checks.
  (upgrade_txns_to_log_addressing): Return a proper "skip" error
                                    when skipping this test.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547470 13f79535-47bb-0310-9956-ffa450edef68