Add test for Java 8 Compatibility of third party jars (#1618)

* Add test for Java 8 Compatibility

Add a test to check for Java 8 compatibility of jars deployed to
AppEngine.

It is not enough to run existing tests with Java 8 VM, since many API
jars are not exercised by tests. For example, those for GCP services
like the SecretManager.

We take the conservative approach and verify that every class in every
jar are compiled for Java 8.

java8compatibility/build.gradle

@@ -0,0 +1,41 @@
+// Copyright 2022 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+configurations {
+    all_prod_jars
+}
+
+dependencies {
+    def deps = rootProject.dependencyMap
+
+    testCompile deps['com.google.guava:guava']
+    testCompile deps['com.google.truth:truth']
+    testCompile deps['org.junit.jupiter:junit-jupiter-api']
+    testCompile deps['org.junit.jupiter:junit-jupiter-engine']
+    testCompile deps['org.junit.jupiter:junit-jupiter-params']
+    testRuntime project(path: ':core', configuration: 'deploy_jar')
+
+    // Collection of jars that get deployed to Appengine. Please refer to
+    // ../appengine_war.gradle for more information.
+    // This includes nomulus production classes and all dependencies.
+    all_prod_jars project(path: ':core', configuration: 'deploy_jar')
+}
+
+test {
+    def jarlist = configurations.all_prod_jars.each {
+        it.toURI().toURL()
+    }.join(',')
+
+    systemProperty 'all_prod_jars', jarlist
+}