CI: Split the config check from the actual test.

I'm using selftest.py to check whether I've installed everything.
Pytest actually finds and runs the tests.  For some reason that wasn't
running earlier.

.github/workflows/test-cygwin.yml

@@ -45,7 +45,9 @@ jobs:
       - name: Install Pillow
         run: |
           C:\tools\cygwin\bin\dash -c '/usr/bin/python3.8 -m pip install dist/*.whl'
-      - name: Test Pillow
+      - name: Check Pillow configuration
         run: |
           C:\tools\cygwin\bin\python3.8 selftest.py
+      - name: Test Pillow
+        run: |
           C:\tools\cygwin\bin\xvfb-run -s '-screen 0 1024x768x24' .ci/test.sh