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/supplementary-groups.sh

@@ -0,0 +1,37 @@
+source common.sh
+
+requireSandboxSupport
+[[ $busybox =~ busybox ]] || skipTest "no busybox"
+if ! command -p -v unshare; then skipTest "Need unshare"; fi
+needLocalStore "The test uses --store always so we would just be bypassing the daemon"
+
+unshare --mount --map-root-user bash <<EOF
+  source common.sh
+
+  # Avoid store dir being inside sandbox build-dir
+  unset NIX_STORE_DIR
+  unset NIX_STATE_DIR
+
+  setLocalStore () {
+    export NIX_REMOTE=\$TEST_ROOT/\$1
+    mkdir -p \$NIX_REMOTE
+  }
+
+  cmd=(nix-build ./hermetic.nix --arg busybox "$busybox" --arg seed 1 --no-out-link)
+
+  # Fails with default setting
+  # TODO better error
+  setLocalStore store1
+  expectStderr 1 "\${cmd[@]}" | grepQuiet "unable to start build process"
+
+  # Fails with `require-drop-supplementary-groups`
+  # TODO better error
+  setLocalStore store2
+  NIX_CONFIG='require-drop-supplementary-groups = true' \
+    expectStderr 1 "\${cmd[@]}" | grepQuiet "unable to start build process"
+
+  # Works without `require-drop-supplementary-groups`
+  setLocalStore store3
+  NIX_CONFIG='require-drop-supplementary-groups = false' \
+    "\${cmd[@]}"
+EOF