Fixing an intermittent test failure (#306)

Author: hiranya911

src/test/java/com/google/firebase/FirebaseAppTest.java

@@ -39,6 +39,7 @@
 import com.google.common.base.Suppliers;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.firebase.FirebaseApp.TokenRefresher;
 import com.google.firebase.FirebaseOptions.Builder;
 import com.google.firebase.database.FirebaseDatabase;
@@ -60,6 +61,7 @@
 import java.util.UUID;
 import java.util.concurrent.Callable;
 import java.util.concurrent.TimeUnit;
+import org.junit.AfterClass;
 import org.junit.Assert;
 import org.junit.BeforeClass;
 import org.junit.Rule;
@@ -83,6 +85,11 @@ public static void setupClass() throws IOException {
     TestUtils.getApplicationDefaultCredentials();
   }
 
+  @AfterClass
+  public static void tearDownClass() {
+    TestUtils.unsetEnvironmentVariables(ImmutableSet.of(FirebaseApp.FIREBASE_CONFIG_ENV_VAR));
+  }
+
   @Test(expected = NullPointerException.class)
   public void testNullAppName() {
     FirebaseApp.initializeApp(OPTIONS, null);