Relative path flakes ("subflakes") are basically fundamentally
broken, since they produce lock file entries like
"locked": {
"lastModified": 1,
"narHash": "sha256-/2tW9SKjQbRLzfcJs5SHijli6l3+iPr1235zylGynK8=",
"path": "./flakeC",
"type": "path"
},
that don't specify what "./flakeC" is relative to. They *sometimes*
worked by accident because the `narHash` field allowed
`fetchToStore()` to get the store path of the subflake *if* it
happened to exist in the local store or in a substituter.
Subflakes are properly fixed in #10089 (which adds a "parent" field to
the lock file). Rather than come up with some crazy hack to make them
work in the interim, let's just disable the only test that depends on
the broken behaviour for now.
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/
```
