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/shell-hello.nix

@@ -0,0 +1,26 @@
+with import ./config.nix;
+
+{
+  hello = mkDerivation {
+    name = "hello";
+    outputs = [ "out" "dev" ];
+    meta.outputsToInstall = [ "out" ];
+    buildCommand =
+      ''
+        mkdir -p $out/bin $dev/bin
+
+        cat > $out/bin/hello <<EOF
+        #! ${shell}
+        who=\$1
+        echo "Hello \''${who:-World} from $out/bin/hello"
+        EOF
+        chmod +x $out/bin/hello
+
+        cat > $dev/bin/hello2 <<EOF
+        #! ${shell}
+        echo "Hello2"
+        EOF
+        chmod +x $dev/bin/hello2
+      '';
+  };
+}