iotests: Remove a few tests from 'quick' group

A run of './check -qcow2 -g quick' on my machine produced only
two tests that took longer than 5 seconds; 178 took 18, and
189 took 7.  Remove them from the quick group.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/group

@@ -175,7 +175,7 @@
 175 auto quick
 176 rw auto backing
 177 rw auto quick
-178 auto quick
+178 auto
 179 rw auto quick
 181 rw auto migration
 182 rw auto quick
@@ -183,4 +183,4 @@
 185 rw auto
 186 rw auto
 188 rw auto quick
-189 rw auto quick
+189 rw auto