Fix flaky MultipartEncoderTckTest.required_spec101... (#5136)

Motivation:

`MultipartEncoderTckTest.required_spec101_XXX` fails quite often.

https://ge.armeria.dev/scans/tests?search.timeZoneId=Asia/Seoul&tests.container=com.linecorp.armeria.common.multipart.MultipartEncoderTckTest&tests.test=required_spec101_subscriptionRequestMustResultInTheCorrectNumberOfProducedElements
```java
java.lang.AssertionError: Publisher com.linecorp.armeria.common.multipart.MultipartEncoder@3eec8583 produced no element after first `request` within 200 ms
at org.testng.Assert.fail(Assert.java:98)
at org.reactivestreams.tck.TestEnvironment.flopAndFail(TestEnvironment.java:285)
at org.reactivestreams.tck.TestEnvironment$Receptacle.next(TestEnvironment.java:1047)
at org.reactivestreams.tck.TestEnvironment$ManualSubscriber.nextElement(TestEnvironment.java:514)
at org.reactivestreams.tck.TestEnvironment$ManualSubscriber.nextElement(TestEnvironment.java:510)
at org.reactivestreams.tck.PublisherVerification$3.run(PublisherVerification.java:212)
at org.reactivestreams.tck.PublisherVerification.activePublisherTest(PublisherVerification.java:1135)
at
org.reactivestreams.tck.PublisherVerification.required_spec101_subscriptionRequestMustResultInTheCorrectNumberOfProducedElements(PublisherVerification.java:204)
```

The failure couldn't be reproduced locally. I checked the log on the
Gradle scan and it shows that the classes are initialized when the test
starts.

I guess JVM lazy class initialization and the Netty worker group
initialization could affect the failure.

Modifications:

- Early initialize Netty worker group before starting the tests
- Increase timeout

Result:

- Closes #4554

---------

Co-authored-by: Trustin Lee <[email protected]>

core/src/test/java/com/linecorp/armeria/common/multipart/MultipartEncoderTckTest.java

@@ -39,6 +39,7 @@
 import org.reactivestreams.tck.TestEnvironment;
 import org.testng.annotations.Test;
 
+import com.linecorp.armeria.common.CommonPools;
 import com.linecorp.armeria.common.HttpData;
 import com.linecorp.armeria.common.stream.StreamMessage;
 
@@ -48,8 +49,13 @@ public class MultipartEncoderTckTest extends PublisherVerification<HttpData> {
 
     // Forked from https://github.com/oracle/helidon/blob/9d209a1a55f927e60e15b061700384e438ab5a01/media/multipart/src/test/java/io/helidon/media/multipart/MultiPartEncoderTckTest.java
 
+    static {
+        // Make sure the worker group is initialized.
+        CommonPools.workerGroup().next().submit(() -> {});
+    }
+
     public MultipartEncoderTckTest() {
-        super(new TestEnvironment(200));
+        super(new TestEnvironment(500));
     }
 
     @Override