Add @RunWith annotations to support non-gradle builds. (#223)

Author: vkryachko

firebase-common/src/test/java/com/google/firebase/VersionGenerationTest.java

@@ -17,7 +17,10 @@
 import static com.google.common.truth.Truth.assertThat;
 
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
+@RunWith(JUnit4.class)
 public class VersionGenerationTest {
 
   @Test

firebase-common/src/test/java/com/google/firebase/platforminfo/DefaultUserAgentPublisherTest.java

@@ -23,7 +23,10 @@
 import java.util.Set;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
+@RunWith(JUnit4.class)
 public class DefaultUserAgentPublisherTest {
   private Set<LibraryVersion> libraryVersions;
   private DefaultUserAgentPublisher userAgentPublisher;

firebase-common/src/test/java/com/google/firebase/platforminfo/OutOfBandLibraryVersionRegistrarTest.java

@@ -17,7 +17,10 @@
 import static com.google.common.truth.Truth.assertThat;
 
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
+@RunWith(JUnit4.class)
 public class OutOfBandLibraryVersionRegistrarTest {
   @Test
   public void registerVersion_persistsVersion() {