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, 29 deletions

diff --git a/tests/lang/eval-okay-delayed-with.nix b/tests/lang/eval-okay-delayed-with.nix
deleted file mode 100644
index 3fb023e1c..000000000
--- a/tests/lang/eval-okay-delayed-with.nix
+++ /dev/null
@@ -1,29 +0,0 @@
-let
-
-  pkgs_ = with pkgs; {
-    a = derivation {
-      name = "a";
-      system = builtins.currentSystem;
-      builder = "/bin/sh";
-      args = [ "-c" "touch $out" ];
-      inherit b;
-    };
-
-    b = derivation {
-      name = "b";
-      system = builtins.currentSystem;
-      builder = "/bin/sh";
-      args = [ "-c" "touch $out" ];
-      inherit a;
-    };
-
-    c = b;
-  };
-
-  packageOverrides = pkgs: with pkgs; {
-    b = derivation (b.drvAttrs // { name = "${b.name}-overridden"; });
-  };
-
-  pkgs = pkgs_ // (packageOverrides pkgs_);
-
-in "${pkgs.a.b.name} ${pkgs.c.name} ${pkgs.b.a.name}"