fstests: rename _require_test_symlinks

Generally these tests are not restricted to one device or the other;
testing the test device will suffice to determine the capability
but it doesn't need to be named that way.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>