qemu-iotests: Remove 091 from quick group

For the purpose of allowing running quick group on tmpfs.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Speed up make check-block

Using /tmp, which is usually mounted as tmpfs, the quick group can be
quicker.

On my laptop (Lenovo T430s with Fedora 20), this reduces the time from
50s to 30s.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
tests/Makefile: Add check-block to make check

Signed-off-by: Fam Zheng <famz@redhat.com>