test: Kill processes that keep scsi_debug mounts busy

Tests like TestStorageUsed.testTeardownRetry run processes that keep a
scsi_debug block device mount busy. If they fail on some assertion in
the middle, the generic storage cleanup (umount, rmmod scsi_debug)
fails, and the following tests get broken. Add an `fuser` kill loop to
prevent that.

Also show all stdout output from these commands. We don't need it
returned in the code, it's more useful for developers in the test
output.

test/common/testlib.py

@@ -1538,10 +1538,12 @@ def cleanup_home_dirs():
                         "for dev in $(ls /sys/bus/pseudo/drivers/scsi_debug/adapter*/host*/target*/*:*/block); do "
                         "    for s in /sys/block/*/slaves/${dev}*; do [ -e $s ] || break; "
                         "        d=/dev/$(dirname $(dirname ${s#/sys/block/})); "
+                        "        while fuser --mount $d --kill; do sleep 0.1; done; "
                         "        umount $d || true; dmsetup remove --force $d || true; "
                         "    done; "
-                        "    umount /dev/$dev 2>/dev/null || true; "
-                        "done; until rmmod scsi_debug; do sleep 0.2; done")
+                        "    while fuser --mount /dev/$dev --kill; do sleep 0.1; done; "
+                        "    umount /dev/$dev || true; "
+                        "done; until rmmod scsi_debug; do sleep 0.2; done", stdout=None)
 
         def terminate_sessions():
             # on OSTree we don't get "web console" sessions with the cockpit/ws container; just SSH; but also, some tests start