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, 28 deletions

diff --git a/tests/plugins/plugintest.cc b/tests/plugins/plugintest.cc
deleted file mode 100644
index e02fd68d5..000000000
--- a/tests/plugins/plugintest.cc
+++ /dev/null
@@ -1,28 +0,0 @@
-#include "config.hh"
-#include "primops.hh"
-
-using namespace nix;
-
-struct MySettings : Config
-{
-    Setting<bool> settingSet{this, false, "setting-set",
-        "Whether the plugin-defined setting was set"};
-};
-
-MySettings mySettings;
-
-static GlobalConfig::Register rs(&mySettings);
-
-static void prim_anotherNull (EvalState & state, const PosIdx pos, Value ** args, Value & v)
-{
-    if (mySettings.settingSet)
-        v.mkNull();
-    else
-        v.mkBool(false);
-}
-
-static RegisterPrimOp rp({
-    .name = "anotherNull",
-    .arity = 0,
-    .fun = prim_anotherNull,
-});