Use custom branch for OFM test

This will look for a branch called `v3-labthings-branch` and fall back to `v3-labthings-main` then `v3` if it doesn't exist.

I've added a line to print the branch that's actually checked out.

Author: rwb27

.github/workflows/test.yml

@@ -178,7 +178,9 @@ jobs:
         run: |
           git clone https://gitlab.com/openflexure/openflexure-microscope-server.git
           cd openflexure-microscope-server
-          git checkout v3
+          git checkout v3-labthings-${{ github.head_ref }} || git checkout v3-labthings-main || git checkout v3
+          git rev-parse --abbrev-ref HEAD
+          git rev-parse HEAD
           pip install -e .[dev]
 
       - name: Install LabThings-FastAPI