[Pytest] Test inability of Monit to reset internal counter and its new syntax as a workaround (#5492)

What is the motivation for this PR?
This PR adds two new test cases to test a Monit issue fixed by the PR (sonic-net/sonic-buildimage#10288) in SONiC image repo.

How did you do it?
Specifically the 'stress' utility is leveraged to increase memory usage of a container continuously, then

Test whether that container can be restarted by the script ran by Monit.

Test whether that container can be restarted by the script ran by Moni; If that container was restarted, then test the script ran
by Monit was unable to restart the container anymore due to Monit failed to reset its internal counter.

Test whether that container can be restarted by the script ran by Moni; If that container was restarted, then test the script ran
by Monit was able to restart the container with the help of new Monit syntax although Monit failed to reset its internal
counter.

How did you verify/test it?
I tested manually against the DuT str-s6000-0n-5 with 20201231.54 and 20191130.83 images. and following is testing results with 20201231.54 image.

Author: yozhao101