chore: add junit imports and annotations for RefreshAheadStrategyTest class (#2196)

Author: kgala2

core/src/test/java/com/google/cloud/sql/core/RefreshAheadStrategyTest.java

@@ -30,7 +30,10 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
+@RunWith(JUnit4.class)
 public class RefreshAheadStrategyTest {
 
   public static final long TEST_TIMEOUT_MS = 3000;