Many folks mount their svn-test-work directories as RAMdisks to speed up the
running of the test suite, and then maybe additionally tell APR (via the
environment) to also use some location on the RAMdisk for tempfile storage. We
should have the test suite create svn-test-work/tmp/ and do the work of telling
APR to use that directory ourselves, which should further help speed up the
tests for those using RAMdisks for svn-test-work, without hurting performance
(in the general case) for those that don't.