[go] Stop enforcing code coverage, stop using *.infra_testing files.

Enforcing code coverage in golang code doesn't seem to bring any positive
feelings and just devolves into a toil of updating *.infra_testing file.

Additionally, go toolset has advanced a lot since 2014. It now longer picks up
stale code from pkg/*, so no need to rebuild dependencies before tests.

As a side effect, this CL now enables tests on all platforms, since
"skip_testing" feature was implemented in *.infra_testing files.

R=iannucci@chromium.org

Change-Id: Id50e645372d70f5ec5c02e9b788e86658729e04d
Reviewed-on: https://chromium-review.googlesource.com/c/1330095
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Cr-Commit-Position: refs/heads/master@{#18920}