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/
```

tests/functional/build-hook.nix

@@ -0,0 +1,62 @@
+{ busybox, contentAddressed ? false }:
+
+with import ./config.nix;
+
+let
+
+  caArgs = if contentAddressed then {
+      outputHashMode = "recursive";
+      outputHashAlgo = "sha256";
+      __contentAddressed = true;
+    } else {};
+
+  mkDerivation = args:
+    derivation ({
+      inherit system;
+      builder = busybox;
+      args = ["sh" "-e" args.builder or (builtins.toFile "builder-${args.name}.sh" ''
+        if [ -e "$NIX_ATTRS_SH_FILE" ]; then source $NIX_ATTRS_SH_FILE; fi;
+        eval "$buildCommand"
+      '')];
+    } // removeAttrs args ["builder" "meta" "passthru"]
+    // caArgs)
+    // { meta = args.meta or {}; passthru = args.passthru or {}; };
+
+  input1 = mkDerivation {
+    shell = busybox;
+    name = "build-remote-input-1";
+    buildCommand = "echo hi-input1; echo FOO > $out";
+    requiredSystemFeatures = ["foo"];
+  };
+
+  input2 = mkDerivation {
+    shell = busybox;
+    name = "build-remote-input-2";
+    buildCommand = "echo hi; echo BAR > $out";
+    requiredSystemFeatures = ["bar"];
+  };
+
+  input3 = mkDerivation {
+    shell = busybox;
+    name = "build-remote-input-3";
+    buildCommand = ''
+      echo hi-input3
+      read x < ${input2}
+      echo $x BAZ > $out
+    '';
+    requiredSystemFeatures = ["baz"];
+  };
+
+in
+
+  mkDerivation {
+    shell = busybox;
+    name = "build-remote";
+    passthru = { inherit input1 input2 input3; };
+    buildCommand =
+      ''
+        read x < ${input1}
+        read y < ${input3}
+        echo "$x $y" > $out
+      '';
+  }