API design for std.TestSetPrevRealm

[thehowl]

Creating this issue following a discussion with @albttx on how to best implement what was started by @r3v4s in #891, std.TestSetPrevRealm.

What Albert and r3v4s are working on a is a TestSetPrevRealm function which would work similarly to the existing TestSetOrigCaller. However, since PrevRealm works on stack frames, for clarity and implementation simplicity, a system like the following might be better:

std.TestWithPrevRealm("gno.land/r/demo/1234", func() {
   // ... 
})

// for instance:

func TestX(t *testing.T) {
	for _, pr := range [...]string{
		"gno.land/r/demo/x",
		"gno.land/r/demo/y",
	} {
		std.TestWithPrevRealm(pr, doXTest(t, pr))
	}
}

const allowedRealm = "gno.land/r/demo/x"

func doXTest(t *testing.T, prev string) func() {
	return func() {
		defer func() {
			r := recover() 
			if (prev == allowedRealm && r != nil) ||
				(prev != allowedRealm && r == nil) {
				t.Errorf("invalid panic %v when calling myrealm.X with caller %q", r, prev)
			}
		}()
		myrealm.X()
	}
}