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/simple.builder.sh

@@ -0,0 +1,11 @@
+echo "PATH=$PATH"
+
+# Verify that the PATH is empty.
+if mkdir foo 2> /dev/null; then exit 1; fi
+
+# Set a PATH (!!! impure).
+export PATH=$goodPath
+
+mkdir $out
+
+echo "Hello World!" > $out/hello