testing: env: refactor SetEnvs

[kamilsk]

Setenv will be available at go 1.17: https://tip.golang.org/pkg/testing/#T.Setenv

pkg/testing/env.go

Lines 46 to 48 in d8517c0

	func SetEnvs(handle func(error), vars ...env.Variable) (func(func(error)), error) {
	return setEnvs(os.LookupEnv, os.Setenv, os.Unsetenv, handle, vars...)
	}

[kamilsk]

also, T.Cleanup is present, so I can extend for example testing.SetEnv to use it, see golang/mock@6d816de