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, 70 deletions

diff --git a/tests/structured-attrs.nix b/tests/structured-attrs.nix
deleted file mode 100644
index e93139a44..000000000
--- a/tests/structured-attrs.nix
+++ /dev/null
@@ -1,70 +0,0 @@
-with import ./config.nix;
-
-let
-
-  dep = mkDerivation {
-    name = "dep";
-    buildCommand = ''
-      mkdir $out; echo bla > $out/bla
-    '';
-  };
-
-in
-
-mkDerivation {
-  name = "structured";
-
-  __structuredAttrs = true;
-
-  outputs = [ "out" "dev" ];
-
-  buildCommand = ''
-    set -x
-
-    [[ $int = 123456789 ]]
-    [[ -z $float ]]
-    [[ -n $boolTrue ]]
-    [[ -z $boolFalse ]]
-    [[ -n ''${hardening[format]} ]]
-    [[ -z ''${hardening[fortify]} ]]
-    [[ ''${#buildInputs[@]} = 7 ]]
-    [[ ''${buildInputs[2]} = c ]]
-    [[ -v nothing ]]
-    [[ -z $nothing ]]
-
-    mkdir ''${outputs[out]} ''${outputs[dev]}
-    echo bar > $dest
-    echo foo > $dest2
-
-    json=$(cat $NIX_ATTRS_JSON_FILE)
-    [[ $json =~ '"narHash":"sha256:1r7yc43zqnzl5b0als5vnyp649gk17i37s7mj00xr8kc47rjcybk"' ]]
-    [[ $json =~ '"narSize":288' ]]
-    [[ $json =~ '"closureSize":288' ]]
-    [[ $json =~ '"references":[]' ]]
-  '';
-
-  buildInputs = [ "a" "b" "c" 123 "'" "\"" null ];
-
-  hardening.format = true;
-  hardening.fortify = false;
-
-  outer.inner = [ 1 2 3 ];
-
-  int = 123456789;
-
-  float = 123.456;
-
-  boolTrue = true;
-  boolFalse = false;
-
-  nothing = null;
-
-  dest = "${placeholder "out"}/foo";
-  dest2 = "${placeholder "dev"}/foo";
-
-  "foo bar" = "BAD";
-  "1foobar" = "BAD";
-  "foo$" = "BAD";
-
-  exportReferencesGraph.refs = [ dep ];
-}