skip TEST(EXT4StoreTest, _detect_fs) if DISK or MOUNTPOINT are undefined

The TEST(EXT4StoreTest, _detect_fs) test is meant to be run from
qa/workunits/filestore/filestore.sh, after the ext4 file system was
created. If the DISK and MOUNTPOINT environment variables are not
defined, display a message explaining the expected environment and
silentely skip the test. The tests in store_test.cc are not unit tests
because they depend on their environment.

http://tracker.ceph.com/issues/5312 fixes #5312

Signed-off-by: Loic Dachary <loic@dachary.org>