Fix shutdown_pipe test issue

[wishdev]

This addresses the test issue from #44. It appears the issue is that the server aborts for an unknown reason. There is no other way the pipe could be closed when the attempt to write to it occurs.

Looking at the test directly above the new one https://github.com/ruby/webrick/blob/1daacc18490cbc96bcf1bdc741f3ccff65a0d164/test/webrick/test_server.rb#L151-L161- it appears that it assumes that a server may not start properly and simply appears to skip the test if the server fails to start. The only way on line 156 that the thread status is a falsy value is if the thread is already dead when it is examined. That would imply the server is non-existent at that point.

The fix tries 10 times to bring up a server and if it fails to do so - it continues on.

[jeremyevans]

For some reasons, CI didn't run all jobs when this was submitted, but after running the jobs, looks like things are working. Thanks!

[wishdev]

As mentioned on #44

1. Error:
   TestWEBrickServer#test_shutdown_pipe:
   IOError: stream closed in another thread
   /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in write' /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in puts'
   /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:180:in block in test_shutdown_pipe' /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in loop'
   /tmp/ruby/v3/src/trunk-no-theap/test/webrick/test_server.rb:167:in `test_shutdown_pipe'
   on high load-average environments.

http://ci.rvm.jp/logfiles/brlog.trunk-no-theap.20200924-222101

So the code above this fix - test_restart_after_stop runs the exact same concept - with the difference being that server.stop swallows this error internally. So it would appear that these tests aren't actually running properly because something is closing the servers (and their shutdown_pipes) prematurely. So I guess I just swallow the IOError here as well?