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 68c81c7375)

tests/functional/user-envs.nix

@@ -0,0 +1,34 @@
+# Some dummy arguments...
+{ foo ? "foo"
+}:
+
+with import ./config.nix;
+
+assert foo == "foo";
+
+let
+
+  platforms = let x = "foobar"; in [ x x ];
+
+  makeDrv = name: progName: (mkDerivation {
+    name = assert progName != "fail"; name;
+    inherit progName system;
+    builder = ./user-envs.builder.sh;
+  } // {
+    meta = {
+      description = "A silly test package with some \${escaped anti-quotation} in it";
+      inherit platforms;
+    };
+  });
+
+in
+
+  [
+    (makeDrv "foo-1.0" "foo")
+    (makeDrv "foo-2.0pre1" "foo")
+    (makeDrv "bar-0.1" "bar")
+    (makeDrv "foo-2.0" "foo")
+    (makeDrv "bar-0.1.1" "bar")
+    (makeDrv "foo-0.1" "foo" // { meta.priority = 10; })
+    (makeDrv "fail-0.1" "fail")
+  ]