Dashboard - Fix flaky create_health_report tests.
Dashboard - Fix flaky create_health_report tests.

They were all manipulating a global instead of copying it.

Bug: chromium:805446
Change-Id: I3220fd7b45508d19e253d346c99979632ca3c048
Reviewed-on: https://chromium-review.googlesource.com/883965
Reviewed-by: Ethan Kuefner <b3258c73b307e35b2725bc68d7434c38b3a2b146@chromium.org>
Commit-Queue: Simon Hatch <490a224d89645d3fea983d7189ce51bde0a00c1d@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult