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, 12 deletions

diff --git a/tests/lang/eval-okay-string.nix b/tests/lang/eval-okay-string.nix
deleted file mode 100644
index 47cc989ad..000000000
--- a/tests/lang/eval-okay-string.nix
+++ /dev/null
@@ -1,12 +0,0 @@
-"foo" + "bar"
-  + toString (/a/b + /c/d)
-  + toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
-  + ("/../foo" + toString /x/y)
-  + "escape: \"quote\" \n \\"
-  + "end
-of
-line"
-  + "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
-  + "foo$bar"
-  + "$\"$\""
-  + "$"