fstests: support check for uuidgen utility

fstests: support check for uuidgen utility

A recent test uses the uuidgen utility to generate UUID-based
filenames, but this package is not necessarily installed as part of
the core packages of every distro.

As such, add the UUIDGEN_PROG environment variable to the common
configuration and update the test to require the existence of
uuidgen to run.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>