When running in QEMU, the 5 ms holding time of mutex_timed_locker may be
too short when the host is busy. This occasionally leads to failing
tests in our CI:

posix-mutex.c:510, FAILED: get_effective_prio() == prio (=false)

Expand to 50 ms in such setups.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>