Put functional tests in `tests/functional`

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/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/functional/lang/eval-fail-to-path.err.exp b/tests/functional/lang/eval-fail-to-path.err.exp
new file mode 100644
index 000000000..43ed2bdfc
--- /dev/null
+++ b/tests/functional/lang/eval-fail-to-path.err.exp
@@ -0,0 +1,14 @@
+error:
+       … while calling the 'toPath' builtin
+
+         at /pwd/lang/eval-fail-to-path.nix:1:1:
+
+            1| builtins.toPath "foo/bar"
+             | ^
+            2|
+
+       … while evaluating the first argument passed to builtins.toPath
+
+         at «none»:0: (source not available)
+
+       error: string 'foo/bar' doesn't represent an absolute path