Backport various post 20220527 release runtime fixes:
* The concept of max runtime
* tst_test.sh: Cleanup getopts usage
* mountns0[1-3]: wait for umount completed in thread_b
Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>