tests/functional: add memlock tests

tests/functional: add memlock tests

Add new tests to check the correctness of the `-overcommit memlock`
option (possible values: off, on, on-fault) by using
`/proc/{qemu_pid}/status` file to check in VmSize, VmRSS and VmLck
values:

* if `memlock=off`, then VmLck = 0;
* if `memlock=on`, then VmLck > 0 and almost all memory is resident;
* if `memlock=on-fault`, then VmLck > 0 and only few memory is resident.

Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
Message-ID: <20250605065908.299979-3-dtalexundeer@yandex-team.ru>
Signed-off-by: Thomas Huth <thuth@redhat.com>