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/export-graph.nix

@@ -0,0 +1,29 @@
+with import ./config.nix;
+
+rec {
+
+  printRefs =
+    ''
+      echo $exportReferencesGraph
+      while read path; do
+          read drv
+          read nrRefs
+          echo "$path has $nrRefs references"
+          echo "$path" >> $out
+          for ((n = 0; n < $nrRefs; n++)); do read ref; echo "ref $ref"; test -e "$ref"; done
+      done < refs
+    '';
+
+  foo."bar.runtimeGraph" = mkDerivation {
+    name = "dependencies";
+    builder = builtins.toFile "build-graph-builder" "${printRefs}";
+    exportReferencesGraph = ["refs" (import ./dependencies.nix {})];
+  };
+
+  foo."bar.buildGraph" = mkDerivation {
+    name = "dependencies";
+    builder = builtins.toFile "build-graph-builder" "${printRefs}";
+    exportReferencesGraph = ["refs" (import ./dependencies.nix {}).drvPath];
+  };
+
+}