Build and test example projects as part of CI build

This would have flagged in recent changes that the sample projects were not all updated for JUnit 5.5.x support.

.travis.yml

@@ -1,13 +1,26 @@
 language: java
 jdk: openjdk11
 
-script: "./gradlew build --scan"
-deploy:
-  provider: script
-  on:
-    tags: true
-  script: "./gradlew publishPlugins"
-  skip_cleanup: true
+jobs:
+  include:
+    - stage: "Build and test plugin"
+      script: "./gradlew build --scan"
+    - stage: "Test examples"
+      name: "test-project"
+      script: "./gradlew -b test-project/build.gradle build"
+    - script: "./gradlew -b test-project-groovy/build.gradle build"
+      name: "test-project-groovy"
+    - script: "./gradlew -b test-project-kotlin/build.gradle.kts build"
+      name: "test-project-kotlin"
+    - script: "./gradlew -b test-project-mixed/build.gradle build"
+      name: "test-project-mixed"
+    - stage: "Deploy"
+      deploy:
+        provider: script
+        on:
+          tags: true
+        script: "./gradlew publishPlugins"
+        skip_cleanup: true
 
 before_install:
   - echo -e "gradle.publish.key=$GRADLE_PUBLISH_KEY\ngradle.publish.secret=$GRADLE_PUBLISH_SECRET" > gradle.properties