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 68c81c7375)

tests/functional/plugins/plugintest.cc

@@ -0,0 +1,28 @@
+#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,
+});