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/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 0 insertions, 22 deletions

diff --git a/tests/lang/eval-okay-attrs3.nix b/tests/lang/eval-okay-attrs3.nix
deleted file mode 100644
index f29de11fe..000000000
--- a/tests/lang/eval-okay-attrs3.nix
+++ /dev/null
@@ -1,22 +0,0 @@
-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"