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-attrs3.nix

@@ -0,0 +1,22 @@
+let
+
+  config = 
+    {
+      services.sshd.enable = true;
+      services.sshd.port = 22;
+      services.httpd.port = 80;
+      hostName = "itchy";
+      a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z = "x";
+      foo = {
+        a = "a";
+        b.c = "c";
+      };
+    };
+
+in
+  if config.services.sshd.enable
+  then "foo ${toString config.services.sshd.port} ${toString config.services.httpd.port} ${config.hostName}"
+       + "${config.a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z}"
+       + "${config.foo.a}"
+       + "${config.foo.b.c}"
+  else "bar"