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/recursive.sh

@@ -0,0 +1,19 @@
+source common.sh
+
+enableFeatures 'recursive-nix'
+restartDaemon
+
+clearStore
+
+rm -f $TEST_ROOT/result
+
+export unreachable=$(nix store add-path ./recursive.sh)
+
+NIX_BIN_DIR=$(dirname $(type -p nix)) nix --extra-experimental-features 'nix-command recursive-nix' build -o $TEST_ROOT/result -L --impure --file ./recursive.nix
+
+[[ $(cat $TEST_ROOT/result/inner1) =~ blaat ]]
+
+# Make sure the recursively created paths are in the closure.
+nix path-info -r $TEST_ROOT/result | grep foobar
+nix path-info -r $TEST_ROOT/result | grep fnord
+nix path-info -r $TEST_ROOT/result | grep inner1