Document how to test against an unreleased O2

This adds the sentinel file and the instructions for the two escape hatches
around the CVMFS-based simulation tests.

- A pull request can pin a published release with a sim-tests-tag line.
- Touching test/needs-o2-dev enables the source-building o2dev check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

test/README.md

@@ -112,3 +112,16 @@ usage: run_tests.sh [--fail-immediately] [--keep-artifacts]
   If also not set, this will be set to HEAD. However, if there are unstaged
   changes, it will left blank.
 ```
+
+## When your change needs an unreleased O2
+
+The `Simulation tests against CVMFS` check runs against a published
+`O2PDPSuite` release, so a change that depends on an unmerged or unreleased O2
+commit cannot pass it. Two escape hatches, in order of preference:
+
+1. If the O2 change is already in a published daily, pin it: add a line
+   `sim-tests-tag: daily-YYYYMMDD-HHMM-1` to the pull request description.
+2. If it is not published anywhere yet, touch `test/needs-o2-dev` with a
+   one-line reason and a link to the O2 pull request. That enables
+   `build/O2DPG/sim/o2dev`, which builds O2 from source against `dev`. It is
+   much slower, so it is opt-in.

test/needs-o2-dev

@@ -0,0 +1,5 @@
+Touch this file in a pull request that cannot be tested against a published
+O2PDPSuite release because it depends on an unreleased change in O2.
+Replace this text with a one-line reason and a link to the O2 pull request.
+Touching it enables the build/O2DPG/sim/o2dev check, which builds O2 from
+source against AliceO2Group/AliceO2 dev.