This test insisted on placing profiles in NIX_STATE_DIR, but all
packages were removed from the profile immediately after so they did not
act as garbage collector roots. Switch to directly calling nix-build,
allowing the test to run in VMs without NIX_STATE_DIR.
Due to a typo in quoteRegexChars, finding runtime garbage collection roots
was failing on paths that contained a dot, or any other regex chars that would
have to be replaced.
When fixing that error, also add tests to make sure gc continues to
work.
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
