This makes flako usable on Windows using standard test names.
A workaround to bisecting to failures before this CL is to replace the
test_name by something like "mjsunit/regress\regress-1138075", i.e.
using this on the command line to trigger flako:
-p 'test_name="mjsunit/regress\\regress-1138075"'

R=liviurau@chromium.org

Bug: v8:9218
Change-Id: I37596efcaeca780eeacb27c2841fe8302ddb1e49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081610
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76222}