Migrate testsuite-shading tests to JUnit 5 (netty#11323)

Motivation:

JUnit 5 is more expressive, extensible, and composable in many ways, and it's better able to run tests in parallel.

Modifications:

Use JUnit5 in tests

Result:

Related to netty#10757

testsuite-shading/src/test/java/io/netty/testsuite/shading/ShadingIT.java

@@ -16,11 +16,12 @@
 package io.netty.testsuite.shading;
 
 import io.netty.util.internal.PlatformDependent;
-import org.junit.Test;
-import org.junit.Assume;
+import org.junit.jupiter.api.Test;
 
 import java.lang.reflect.Method;
 
+import static org.junit.jupiter.api.Assumptions.assumeFalse;
+
 public class ShadingIT {
 
     private static final String SHADING_PREFIX = System.getProperty("shadingPrefix2");
@@ -29,7 +30,7 @@ public class ShadingIT {
     @Test
     public void testShadingNativeTransport() throws Exception {
         // Skip on windows.
-        Assume.assumeFalse(PlatformDependent.isWindows());
+        assumeFalse(PlatformDependent.isWindows());
 
         String className = PlatformDependent.isOsx() ?
                 "io.netty.channel.kqueue.KQueue" : "io.netty.channel.epoll.Epoll";
@@ -40,7 +41,7 @@ public void testShadingNativeTransport() throws Exception {
     @Test
     public void testShadingTcnative() throws Exception {
         // Skip on windows.
-        Assume.assumeFalse(PlatformDependent.isWindows());
+        assumeFalse(PlatformDependent.isWindows());
 
         String className = "io.netty.handler.ssl.OpenSsl";
         testShading0(SHADING_PREFIX, className);