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

Reland of the commit https://chromium-review.googlesource.com/c/1330095

The only difference is cleaning of GOBIN before running tests. Otherwise
tests mistakenly pick up stray binaries (like 'git') from there, since it is
in PATH.

Previously GOBIN was cleaned as a byproduct of 'go clean', which we no longer
call.

R=tandrii@chromium.org, nodir@chromium.org

Change-Id: I1da9458a7c1739c07aee319d10b5e4d0f52ff5c9
Reviewed-on: https://chromium-review.googlesource.com/c/1330974
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#18931}