[integration-test] Load test proc mount in a workspace

[csweichel]

Is your feature request related to a problem? Please describe

On several occasions we have broken Docker containers in a workspace due to intermittently failing proc mounts, e.g. due to libseccomp issues (see #7793). We've seen reports that the problem seems to persist #8070.

We must add an integration test which runs the proc stress test (mount /proc in a mount namespace) to detect this issue before bringing it to prod.

[kylos101]

Workspace Team, Once this is implemented, assuming you can now recreate proc mount failures roughly 10% of the time, please resume work on #8070.