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, 25 deletions

diff --git a/tests/search.nix b/tests/search.nix
deleted file mode 100644
index fea6e7a7a..000000000
--- a/tests/search.nix
+++ /dev/null
@@ -1,25 +0,0 @@
-with import ./config.nix;
-
-{
-  hello = mkDerivation rec {
-    name = "hello-${version}";
-    version = "0.1";
-    buildCommand = "touch $out";
-    meta.description = "Empty file";
-  };
-  foo = mkDerivation rec {
-    name = "foo-5";
-    buildCommand = ''
-      mkdir -p $out
-      echo ${name} > $out/${name}
-    '';
-  };
-  bar = mkDerivation rec {
-    name = "bar-3";
-    buildCommand = ''
-      echo "Does not build successfully"
-      exit 1
-    '';
-    meta.description = "broken bar";
-  };
-}