fix check for rw mounts test

[haircommander]

sometime today, the test privileged ctr -- check for rw mounts started failing in integration_{fedora,rhel} because /sys/fs/cgroup was not mounted ro by runc. Oddly, the test is named "check for rw mounts", but switching the grep line to rw makes it fail with crun.

instead of tracking down the source of the issue, I've made the test nearly no-op, by just catting /proc/mounts here

We should determine what is expected by that test, and figure out which runtime is wrong.

[kolyshkin]

Alas, the test itself is wrong. It is supposed to run a privileged container, and commit d3a50d8 (PR #2627) even adds the test/testdata/container_config_privileged.json file, but it is not used by the test.

I am looking into it.