build: run make test during CI workflow

Tests for examples exist, however they were not being run as part of the CI.
This commit fixes it by adding a new step in the test workflow which runs said
tests.

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: Test Coverage
       run: make test-coverage
+
+    - name: Examples
+      env:
+        EXAMPLES_DIR: _examples
+      run: go test -test.v -test.run='^TestBuildExamples$'