ci: run examples tests

Tests for examples exist and are run as part of the Makefile test
target, however they were not being run as part of the CI. This commit
fixes it by adding a new step in the test workflow.

Related to go-git#912.

.github/workflows/test.yml

@@ -32,5 +32,10 @@ jobs:
         git config --global user.email "gha@example.com"
         git config --global user.name "GitHub Actions"
 
-    - name: Test
+    - name: Coverage
       run: make test-coverage
+
+    - name: Examples
+      env:
+        EXAMPLES_DIR: _examples
+      run: go test -v _examples/common_test.go _examples/common.go --examples