Move MIMA checks to a separate CI job

[anatoliykmetyuk]

Currently, some of the PRs labelled as needs-minor-release don't pass the CI since they don't properly exclude their changes from MIMA checks. Since this is the case, there is a tendency to automatically blame a CI failure at first glance on MIMA misconfiguration. This is potentially harmful since a legitimate test failure might get into the compiler if a PR that fails the CI and is assumed to do so because of MIMA gets merged.

So, we should move the MIMA checks from the test CI job to a separate job so that if it is immediately clear at first glance if the failure is MIMA or not.