7903838: apidiff: Update Vintage JUnit test

Reviewed-by: cstein, iris

Author: jonathan-gibbons

test/junit/apitest/SerializationMethodsTest.java

@@ -35,7 +35,7 @@
 
 import jdk.codetools.apidiff.Main;
 
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;