Test against feature branch if requested

It's now possible to add a line to the PR description that starts with "OFM Feature Branch:". This specifies a feature branch to test against.

There's some CI logic to:
* Run the job only if a matching line is present
* Extract the matching line from the PR description
* Check out the relevant branch.

Author: rwb27

.github/workflows/test.yml

@@ -159,7 +159,7 @@ jobs:
           send-summary-comment: true
           show-annotations: "warning"
 
-  test-against-ofm:
+  test-against-ofm-v3:
     runs-on: ubuntu-latest
     continue-on-error: true
     defaults:
@@ -178,9 +178,7 @@ jobs:
         run: |
           git clone https://gitlab.com/openflexure/openflexure-microscope-server.git
           cd openflexure-microscope-server
-          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
+          git checkout v3
           pip install -e .[dev]
 
       - name: Install LabThings-FastAPI
@@ -208,3 +206,66 @@ jobs:
 
       - name: Type check with `mypy`
         run: mypy src
+
+  test-against-ofm-feature-branch:
+    # This job runs only if a feature branch is specified in the merge request description.
+    # The line below looks for a line starting with `OFM Feature Branch:`. This should
+    # match the `grep` command in the relevant step.
+    if: contains(toJson(github.event.pull_request.body), '\r\nOFM Feature Branch:')
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    defaults:
+      run:
+        working-directory: /home/runner/work/openflexure-microscope-server/
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: 3.11
+
+      - name: Clone OpenFlexure Microscope Server
+        working-directory: /home/runner/work/
+        run: git clone https://gitlab.com/openflexure/openflexure-microscope-server.git
+      
+      - name: Checkout feature branch
+        env:
+          PULL_REQUEST_BODY: ${{ github.event.pull_request.body }}
+        # The `if:` block for this job has already checked we will have a matching line.
+        # The logic below will first extract the matching line from the PR description,
+        # then remove the prefix so we're left with only the branch name, which we check
+        # out.
+        run: |
+          matching_line=$(echo "${PULL_REQUEST_BODY}" | grep "^OFM Feature Branch:")
+          feature_branch="${matching_line##"OFM Feature Branch: "}"
+          git checkout "${feature_branch}"
+      
+      - name: Install OpenFlexure Microscope Server
+        run: pip install -e .[dev]
+
+      - name: Install LabThings-FastAPI
+        run: pip install -e ../labthings-fastapi/labthings-fastapi/
+
+      - name: Print installed packages
+        run: pip freeze
+      
+      - name: Configure Git identity
+        run: |
+          git config --global user.name "Sir Unit of Test"
+          git config --global user.email "bogus@email.com"
+      
+      - name: Pull OFM web app
+        run: python ./pull_webapp.py
+      
+      - name: Run OFM unit tests
+        run: pytest
+
+      - name: Run OFM integration tests
+        run: pytest tests/integration_tests
+
+      - name: Run OFM lifecycle test
+        run: tests/lifecycle_test/testfile.py
+      
+      - name: Type check with `mypy`
+        run: mypy src