upload_prebuilts_unittest: convert mox to mock

BUG=chromium:430770
TEST=`./cbuildbot/run_tests` passes

Change-Id: I144655d801439c806ff91635ebdf87772eafbbdb
Reviewed-on: https://chromium-review.googlesource.com/265344
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed