Update BracketCheckerTest.java

exercises/bracket-push/src/test/java/BracketCheckerTest.java

 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
-class BracketCheckerTest {
+public class BracketCheckerTest {
 
     @Test
     public void testPairedSquareBrackets() {