test587: add a slight delay after test

This test is designed to connect to the server, then immediately send a
few bytes and disconnect. In some situations, such as on a loaded
server, this doesn't give the server enough time to write its lock file
before its existence is checked. The test harness then fails to find the
server's input log file (because it hasn't been written yet) and fails
the test. By adding a short delay after the test, the HTTP server has
enough time to write its lock file which gives itself more time to write
its remaining files.

Ref: #11328

tests/data/test587

@@ -31,7 +31,9 @@ lib%TESTNUMBER
  <name>
 HTTP multi-part formpost with aborted read callback
  </name>
- <command>
+# Add a delay to give server time to write its output since we otherwise might
+# not even give it enough time to write its lock file.
+ <command delay="1">
 http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>