t/httpd-corner: improve reliability and diagnostics
The graceful-shutdown-on-PUT test is unreliable because we can't
rely on a FIFO as we do with the GET tests. So increase the
delay to 100ms since that seems enough on my system even with
CONFIG_HZ=100.
Add a timeout and backtrace to the $check_self sub to help with
further diagnostics while we're at it, too.
It would be nice if there were a portable syscall tracing
mechanism we could attach to the -httpd process to make the test
more determistic...