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/
```

tests/functional/lang/eval-okay-foldlStrict-lazy-initial-accumulator.nix

@@ -0,0 +1,6 @@
+# Checks that the nul value for the accumulator is not forced unconditionally.
+# Some languages provide a foldl' that is strict in this argument, but Nix does not.
+builtins.foldl'
+  (_: x: x)
+  (throw "This is never forced")
+  [ "but the results of applying op are" 42 ]