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/ssh-relay.sh

@@ -0,0 +1,16 @@
+source common.sh
+
+echo foo > $TEST_ROOT/hello.sh
+
+ssh_localhost=ssh://localhost
+remote_store=?remote-store=$ssh_localhost
+
+store=$ssh_localhost
+
+store+=$remote_store
+store+=$remote_store
+store+=$remote_store
+
+out=$(nix store add-path --store "$store" $TEST_ROOT/hello.sh)
+
+[ foo = $(< $out) ]