[milo] fix TestShouldUpdateBuilderSummary
[milo] fix TestShouldUpdateBuilderSummary

Ensure parallel calls always happen in the same time bucket so the
test result doesn't depend on when we run the test.

R=mdraz, nqmtuan

Bug: 1304383
Change-Id: Ib1cead6446db37f8111b00a78d1fdba28a088605
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3532759
Reviewed-by: Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Auto-Submit: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

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