Set encoding for java tests

The java compiler in the builder with new base image defaults to ASCII encoding.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=242726695

gradle/build.gradle

@@ -263,6 +263,8 @@ subprojects {
   targetCompatibility = '1.8'
 
   compileJava { options.encoding = "UTF-8" }
+  compileTestJava { options.encoding = "UTF-8" }
+
 
   gradleLint.rules = [
       // Checks if Gradle wrapper is up-to-date