drcachesim pipe-using tests can get into an all-fail state with stale pipe files

*Tests failing* (first 5)
code_api|linux.signest (http://dynamorio.org/CDash/testDetails.php?test=4183&build=13022)
code_api|tool.drcachesim.simple (http://dynamorio.org/CDash/testDetails.php?test=133170&build=13022)
code_api|tool.drcachesim.TLB-simple (http://dynamorio.org/CDash/testDetails.php?test=133171&build=13022)
code_api|tool.drcachesim.phys (http://dynamorio.org/CDash/testDetails.php?test=133172&build=13022)
code_api|tool.drcachesim.thumb (http://dynamorio.org/CDash/testDetails.php?test=133173&build=13022)

All the pipe ones failed:

failed to read from drtestpipe1
ERROR: failed to run analyzer

These passed in the prior build. They also pass on my ARM chromebook.