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, 77 deletions

diff --git a/tests/check-refs.nix b/tests/check-refs.nix
deleted file mode 100644
index 89690e456..000000000
--- a/tests/check-refs.nix
+++ /dev/null
@@ -1,77 +0,0 @@
-with import ./config.nix;
-
-rec {
-
-  dep = import ./dependencies.nix {};
-
-  makeTest = nr: args: mkDerivation ({
-    name = "check-refs-" + toString nr;
-  } // args);
-
-  src = builtins.toFile "aux-ref" "bla bla";
-
-  test1 = makeTest 1 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $dep $out/link";
-    inherit dep;
-  };
-
-  test2 = makeTest 2 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s ${src} $out/link";
-    inherit dep;
-  };
-
-  test3 = makeTest 3 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $dep $out/link";
-    allowedReferences = [];
-    inherit dep;
-  };
-
-  test4 = makeTest 4 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $dep $out/link";
-    allowedReferences = [dep];
-    inherit dep;
-  };
-
-  test5 = makeTest 5 {
-    builder = builtins.toFile "builder.sh" "mkdir $out";
-    allowedReferences = [];
-    inherit dep;
-  };
-
-  test6 = makeTest 6 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $out $out/link";
-    allowedReferences = [];
-    inherit dep;
-  };
-
-  test7 = makeTest 7 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $out $out/link";
-    allowedReferences = ["out"];
-    inherit dep;
-  };
-
-  test8 = makeTest 8 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s ${test1} $out/link";
-    inherit dep;
-  };
-
-  test9 = makeTest 9 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; ln -s $dep $out/link";
-    inherit dep;
-    disallowedReferences = [dep];
-  };
-
-  test10 = makeTest 10 {
-    builder = builtins.toFile "builder.sh" "mkdir $out; echo $test5; ln -s $dep $out/link";
-    inherit dep test5;
-    disallowedReferences = [test5];
-  };
-
-  test11 = makeTest 11 {
-    __structuredAttrs = true;
-    unsafeDiscardReferences.out = true;
-    outputChecks.out.allowedReferences = [];
-    buildCommand = ''echo ${dep} > "''${outputs[out]}"'';
-  };
-
-}