Close ServerContext before Executor in AbstractNettyHttpServerTest

[idelpivnitskiy]

Motivation:

Because the server's executor is closed before the server, all tests
from AbstractNettyHttpServerTest subclasses print
TaskBasedSignalOffloader - Failed to execute task on the executor
errors that pollute logs and make the build log unreadable.

Modifications:

• Close ServerContext before Executor in AbstractNettyHttpServerTest;

Result:

No TaskBasedSignalOffloader errors in test logs.

---

Example of the polluted logs:

io.servicetalk.http.netty.NettyHttpServerConnectionAcceptorTest > testAcceptConnection[ssl=false client=IMMEDIATE server=CACHED DELAY_REJECT_ALL] STANDARD_OUT
    2020-07-22 17:50:53,190       client-io-executor-13-12 [ERROR] TaskBasedSignalOffloader       - Failed to execute task on the executor io.servicetalk.concurrent.api.ContextPreservingStExecutor@5a20367f. Invoking Subscriber (onError()) in the caller thread. Subscriber ContextPreservingCompletableSubscriberAndCancellable(ContextPreservingCompletableSubscriber(io.servicetalk.concurrent.api.ResumeCompletable$ResumeSubscriber@193d4b0c)).
    java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@1b1d3c44 rejected from java.util.concurrent.ThreadPoolExecutor@1f22abff[Shutting down, pool size = 1, active threads = 0, queued tasks = 0, completed tasks = 1]
        at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063) ~[?:1.8.0_252]
        at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830) ~[?:1.8.0_252]
        at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379) ~[?:1.8.0_252]
        at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112) ~[?:1.8.0_252]
...

[idelpivnitskiy]

jdk8 build failed due to #1102

@servicetalk-bot test this please