Remove diagnostic test step from CI workflow

The diagnostic test was blocking the full test run from executing. Since
we know tests will fail due to the vplanet "(null)" units issue (a known
limitation in public vplanet), we should run all tests to see which ones
pass and get full coverage data.

Removing the diagnostic step allows the full test suite to run and provides
better visibility into which tests are affected by the vplanet issue versus
other potential problems.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: RoryBarnes

.github/workflows/tests.yml

@@ -49,13 +49,6 @@ jobs:
               popd
           done
 
-      - name: Run diagnostic test
-        timeout-minutes: 3
-        run: |
-          # First run a simple unit test to verify pytest works
-          echo "Running single unit test as diagnostic..."
-          python -m pytest tests/test_figure.py::test_basic -v -s
-
       - name: Enable subprocess coverage
         run: |
           # Install coverage subprocess support