Use t.Cleanup to remove temporally directories and set environment variables

[itchyny]

This PR suggests migrating test setup cleanup to use t.Cleanup. This PR removes calling teardown returned by setup functions, also refactors environment variable setups. An existing test fails to remove the temporally directory but using t.Cleanup in test utility prevents a mistake like this. Note that gitconfig.WithConfig would be improved by  calling t.Cleanup inside it.

[coveralls]

Pull Request Test Coverage Report for Build 2288166973

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.009%

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Totals
Change from base Build 2280488901:	0.0%
Covered Lines:	1094
Relevant Lines:	1334

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💛 - Coveralls

[Songmu]

It's good practice to be proactive in using new syntax!