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, 0 insertions, 10 deletions

diff --git a/tests/lang/eval-fail-bad-string-interpolation-3.err.exp b/tests/lang/eval-fail-bad-string-interpolation-3.err.exp
deleted file mode 100644
index ac14f329b..000000000
--- a/tests/lang/eval-fail-bad-string-interpolation-3.err.exp
+++ /dev/null
@@ -1,10 +0,0 @@
-error:
-       … while evaluating a path segment
-
-         at /pwd/lang/eval-fail-bad-string-interpolation-3.nix:1:3:
-
-            1| ''${x: x}''
-             |   ^
-            2|
-
-       error: cannot coerce a function to a string