Test failure: `com.linecorp.armeria.server.ServerTest.unsuccessfulStartupTerminatesBossGroup()`

```
org.opentest4j.AssertionFailedError: 
expected: 1L
 but was: 2L
	at app//com.linecorp.armeria.server.ServerTest.unsuccessfulStartupTerminatesBossGroup(ServerTest.java:245)
	at java.base@11.0.29/java.lang.reflect.Method.invoke(Method.java:566)
	at java.base@11.0.29/java.util.ArrayList.forEach(ArrayList.java:1541)
	at java.base@11.0.29/java.util.ArrayList.forEach(ArrayList.java:1541)

```
