fstests: create swap group

I'm going to add a bunch of tests for swap files, so create a group
for them and add the existing tests.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>