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/gc-runtime.nix

@@ -0,0 +1,17 @@
+with import ./config.nix;
+
+mkDerivation {
+  name = "gc-runtime";
+  builder =
+    # Test inline source file definitions.
+    builtins.toFile "builder.sh" ''
+      mkdir $out
+
+      cat > $out/program <<EOF
+      #! ${shell}
+      sleep 10000
+      EOF
+
+      chmod +x $out/program
+    '';
+}