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

This reverts commit 0b5281001766e2d1d041e40dce7f437485fd3b19.

Reason for revert: i believe this CL broke https://luci-milo.appspot.com/p/infra-internal/builders/luci.infra-internal.prod/infra-packager-linux-64
I suspect reinstall_packages is important.

vm63-m0.official:/b/s/c/named/infra_gclient_with_go/infra/go/bin
is not a valid binary

Original change's description:
> [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}

TBR=iannucci@chromium.org,vadimsh@chromium.org,vadimsh@google.com

Change-Id: Ib3e8f858d1afecc89076fae4298b860985112d3c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1330948
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#18926}