Update TestMain() to use os.Exit()

[farazdagi]

What type of PR is this?

Other

What does this PR do? Why is it needed?

• Remove redundant calls to os.exit() in TestMain #7761 updated usage of TestMain()
• However, rules_go does not support that yet: Support go1.15s non Os.Exit TestMain bazelbuild/rules_go#2716
• This PR updates TestMain() back to rely on os.Exit() and fixes tests that weren't discovered due to the issue with rules_go.

Which issues(s) does this PR fix?

N/A

Other notes for review