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

[Juneezee]

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.

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

func TestFoo(t *testing.T) {
	// before
	os.Setenv(key, "new value")
	defer os.Unsetenv(key)
	
	// after
	t.Setenv(key, "new value")
}

This PR also splits large test cases into subtests. This is useful because VSCode is able to run subtest separately

[hairyhenderson]

Hi @Juneezee, thanks for working on this. Most of these tests pre-date Go 1.17, which is why I didn't use t.Setenv (though you'll see that I have started using it in some newer tests in certain packages!).

I would have preferred to see the t.Setenv changes happen separately from the t.Run changes - as it is this is a fairly complex PR to review, and it will take some time, especially given that I want to make sure the test logic is entirely unchanged. But, I will review in depth when I have some time.

[Juneezee]

Hi @hairyhenderson. No worries 😄

[hairyhenderson]

Thanks @Juneezee! I've made some minor adjustments and rebased, otherwise this LGTM.