Test suite for `cabal` the binary should be isolated/independent

Currently, `cabal-testsuite` (which seems to be the main way cabal the binary is tested) cannot be used as a stand-alone binary to run the tests.

The reasons seem to be:

- it requires sources that are part of the repository
- there is a generated module that depends on the current cached `LocalBuildInfo` and hardcodes it into the binary, which then is used to reconstruct arguments to `ghc` when compiling tests
- it's not clear to me whether this is only required for the `Setup.hs` tests or the `cabal-install` binary tests as well (e.g. there is `--skip-setup-tests` but my observation is that this isn't really enough)

The end goal I have is:

- build and test is strictly separated in CI and the test step does not involve building anything other than the tests
- tests can be run on the end users system (ultimately through GHCup)