Upload artifacts showing diff images on test failure (#675)

Tests with @pytest-mpl.mpl_image_compare and @check_figures_equal (#555)
will return an image diff on test failures in the 'tmp-test-dir-with-unique-name'
directory, and we can upload those files as a Github artifact for inspection purposes.

* Use GitHub Action to upload diff images on test failure
* Set a unique artifact name for each OS/Python Version
* Add upload-artifact to GMT Latest tests

Co-Authored-By: Dongdong Tian <seisman.info@gmail.com>

Author: weiji14

.github/workflows/ci_tests.yaml

@@ -117,6 +117,14 @@ jobs:
         shell: bash -l {0}
         run: make test PYTEST_EXTRA="-r P"
 
+      # Upload diff images on test failure
+      - name: Upload diff images if any test fails
+        uses: actions/upload-artifact@v2
+        if: ${{ failure() }}
+        with:
+          name: artifact-${{ runner.os }}-${{ matrix.python-version }}
+          path: tmp-test-dir-with-unique-name
+
       # Build the documentation
       - name: Build the documentation
         shell: bash -l {0}

.github/workflows/ci_tests_dev.yaml

@@ -86,3 +86,11 @@ jobs:
       # Run the tests
       - name: Test with pytest
         run: make test PYTEST_EXTRA="-r P"
+
+      # Upload diff images on test failure
+      - name: Upload diff images if any test fails
+        uses: actions/upload-artifact@v2
+        if: ${{ failure() }}
+        with:
+          name: artifact-GMT-${{ matrix.gmt_git_ref }}-${{ runner.os }}
+          path: tmp-test-dir-with-unique-name