test: use T.Setenv to set env vars in tests

[Juneezee]

A testing cleanup.

This PR replaces os.Setenv with t.Setenv. Starting from Go 1.17, we can use t.Setenv to set environment variable in test. The environment variable is automatically restored to its original value when the test and all its subtests complete. This ensures that each test does not start with leftover environment variables from previous completed tests.

This saves us at least 2 lines (error check, and cleanup) on every instance.

Reference: https://pkg.go.dev/testing#T.Setenv

func TestFoo(t *testing.T) {
	// before
	key := "ENV"
	originalEnv := os.Getenv(key)

	os.Setenv(key, "new value")
	defer os.Setenv(key, originalEnv)
	
	// after
	t.Setenv(key, "new value")
}

[shirou]

Indeed. Awesome contribution!