CI: Move --mpl to default pytest args in run-tests

This eliminates the pytest-mpl piece being a mandatory part of the
action.

.github/actions/run-tests/action.yml

@@ -15,7 +15,7 @@ inputs:
   pytest-args:
     description: 'Additional arguments to pass to pytest'
     required: false
-    default: '-W error::metpy.deprecation.MetpyDeprecationWarning'
+    default: '--mpl -W error::metpy.deprecation.MetpyDeprecationWarning'
 runs:
   using: composite
   steps:
@@ -30,7 +30,7 @@ runs:
       # By running coverage in "parallel" mode and "combining", we can clean up the path names
       run: |
         set -e -o pipefail
-        python -m coverage run -p -m pytest --mpl ${{ inputs.pytest-args }} tests/ 2>&1 | tee tests-${{ inputs.key }}.log
+        python -m coverage run -p -m pytest ${{ inputs.pytest-args }} tests/ 2>&1 | tee tests-${{ inputs.key }}.log
         python -m coverage combine
         python -m coverage report
         python -m coverage xml