skip _check_test_fs unless a test is actually run

skip _check_test_fs unless a test is actually run

Running _check_test_fs can take a non-trivial amount of time, and if a
test has been skipped because it doesn't work on Linux, or it doesn't
work of for ext4, it's pointless to re-run _check_test_fs.  So move
the call to _check_test_fs so it is only run if a test is actually run.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>