Add CI testing for Rocky Linux with integration test, and distinguish rebench return codes

[smarr]

In preparation of #281, this PR adds a Rocky Linux integration test.
Most of our use is on Ubuntu, though, some people use a RHEL-like Linux.
Thus, the key chain is to add CI testing.

However, we also:

• use now different exit codes for different ReBench failure modes
• check in tests whether the CPU info is present. it's not present on Rocky Linux on GitHub Actions
• shielding can also return a string

[coveralls]

coverage: 84.488% (-0.01%) from 84.499%
when pulling 23c3670 on test-on-rocky
into d24a4c4 on master.