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/flakes/config.sh

@@ -0,0 +1,42 @@
+source common.sh
+
+cp ../simple.nix ../simple.builder.sh ../config.nix $TEST_HOME
+
+cd $TEST_HOME
+
+rm -f post-hook-ran
+cat <<EOF > echoing-post-hook.sh
+#!/bin/sh
+
+echo "ThePostHookRan as \$0" > $PWD/post-hook-ran
+EOF
+chmod +x echoing-post-hook.sh
+
+cat <<EOF > flake.nix
+{
+    nixConfig.post-build-hook = ./echoing-post-hook.sh;
+    nixConfig.allow-dirty = false; # See #5621
+
+    outputs = a: {
+       packages.$system.default = import ./simple.nix;
+    };
+}
+EOF
+
+# Without --accept-flake-config, the post hook should not run.
+nix build < /dev/null
+(! [[ -f post-hook-ran ]])
+clearStore
+
+nix build --accept-flake-config
+test -f post-hook-ran || fail "The post hook should have ran"
+
+# Make sure that the path to the post hook doesn’t change if we change
+# something in the flake.
+# Otherwise the user would have to re-validate the setting each time.
+mv post-hook-ran previous-post-hook-run
+echo "# Dummy comment" >> flake.nix
+clearStore
+nix build --accept-flake-config
+diff -q post-hook-ran previous-post-hook-run || \
+    fail "Both post hook runs should report the same filename"