Move /static-checks/diff/removed-rules out of diff + provide shared in lib

[comps]

The removed-rules test doesn't really fit the diff category - all other tests are expected to fail + failures reviewed manually, but removed-rules works more like a traditional regression test, it should never fail.

Moving it outside would mean breaking access to shared (which includes "get me old content" logic), which is why I was reluctant to do that so far.

However we now need other tests with access to that functionality - namely the replacement for Sanity/test-hardening-pkg-update which should not be in diff anyway.

So

1. move the shared functionality to somewhere in lib
2. move removed-rules out of diff, into /static-checks
3. create a replacement for Sanity/test-hardening-pkg-update (in /hardening probably) using the functionality

[comps]

Fixed by #196