Adds a new drcachesim option -subdir_prefix so our tests using the same application can all have unique directory name prefixes, avoiding races on removing and finding output files while testing. We need to store this string into a plain-char buffer at init time to avoid using malloc mid-run.
Updates all of the non-static (plus the L0filter static) drcachesim offline tests to use the new flag, eliminating the need for the existing dependency marking and fixing races in the new tests which were not completely marked versus all the prior tests.
Fixes #3770 (closed)