[cronet] Fix leak of dispatch_semaphore_t in CronetTestBase

The class CronetTestBase allocate a dispatch_semaphore_t to wait
for each task completion but does not destroy those semaphores
in -reset method. This means that there is an ever growing cache
of semaphores (also known as a leak).

Destroy the semaphores when the wait has been successful (as it
won't be possible for the semaphore to be re-signaled until the
main thread get the control back anyway).

Bug: none
Change-Id: I8fc54597066f1662323737fdeb5cabc5285dc03c
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2851237


Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880993}