generic/007: cleanup $TEST_DIR/$seq dir

Test generic/007 was failing after running test overlay/007,
because the latter left behind a non empty directory named 007
in test dir and the former failed to mkdir a directory with the
same name.

Cleanup $TEST_DIR/$seq both at end of overlay/007 and beginning of
generic/007 to avoid this collision.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>