[scheduler] Fix gitiles_test.go.
[scheduler] Fix gitiles_test.go.

It used gomock API incorrectly but not calling its Finish.
As a result gomock had no opportunity to report "your expected
call was not performed".

More recent gomock version calls Finish() automatically and all such
buggy tests are failing now.

R=tandrii@chromium.org, iannucci@chromium.org
BUG=1057067

Change-Id: I0ef9cf147c77cae365e3c339ed6e3f534557f4d3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3018446
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

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