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/lang/eval-okay-attrs5.nix

@@ -0,0 +1,21 @@
+with import ./lib.nix;
+
+let
+
+  as = { x.y.z = 123; a.b.c = 456; };
+
+  bs = { f-o-o.bar = "foo"; };
+
+  or = x: y: x || y;
+  
+in
+  [ as.x.y.z
+    as.foo or "foo"
+    as.x.y.bla or as.a.b.c
+    as.a.b.c or as.x.y.z
+    as.x.y.bla or bs.f-o-o.bar or "xyzzy"
+    as.x.y.bla or bs.bar.foo or "xyzzy"
+    (123).bla or null.foo or "xyzzy"
+    # Backwards compatibility test.
+    (fold or [] [true false false])
+  ]