Try exempting `safe for tests` label from janitor updating `wait for user action` to `untriaged` when unstale.

Looks like this is really meant to exempt that label from the action. Try this out in case this also includes exempting the *removal* of that label from counting as a unstaleness event. This shouldn't hurt since github bot removes the tag after tests run anyways.

Example: https://github.com/protocolbuffers/protobuf/pull/23994
Documentation: https://github.com/actions/stale
PiperOrigin-RevId: 820319199

.github/workflows/janitor.yml

diff --git a/.github/workflows/janitor.yml b/.github/workflows/janitor.yml
index 5b53f25..7f19269 100644
--- a/.github/workflows/janitor.yml
+++ b/.github/workflows/janitor.yml
@@ -90,4 +90,5 @@
           stale-pr-label: 'wait for user action'
           labels-to-add-when-unstale: 'untriaged'
           operations-per-run: 20
+          exempt-pr-labels: 'safe for tests'