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 68c81c7375)

tests/functional/lang/eval-okay-sort.nix

@@ -0,0 +1,20 @@
+with builtins;
+
+[ (sort lessThan [ 483 249 526 147 42 77 ])
+  (sort (x: y: y < x) [ 483 249 526 147 42 77 ])
+  (sort lessThan [ "foo" "bar" "xyzzy" "fnord" ])
+  (sort (x: y: x.key < y.key)
+    [ { key = 1; value = "foo"; } { key = 2; value = "bar"; } { key = 1; value = "fnord"; } ])
+  (sort lessThan [
+    [ 1 6 ]
+    [ ]
+    [ 2 3 ]
+    [ 3 ]
+    [ 1 5 ]
+    [ 2 ]
+    [ 1 ]
+    [ ]
+    [ 1 4 ]
+    [ 3 ]
+  ])
+]