Fix flakiness in a runExclusive test by using channels instead of waits
Fix flakiness in a runExclusive test by using channels instead of waits

This test has been flaky on the Mac and Win trybots (~3 flakes in ~15
days is my best guess).

Bug: 794666
Change-Id: I86995ccf5f2e07c297b41fddbebe7e8ef297a3ec
Reviewed-on: https://chromium-review.googlesource.com/826411
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Charlie Andrews <532d7316e2689bea110898693dc3dc5671811b69@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go