Add test for #48

[jmert]

Adds the tests related to issue #48 and the change made in #49.

I tried making sure these test fail when the change in #48 is reverted, but I couldn't get it to trigger. I don't know if I'm just invoking pkg> test Requires from the wrong environment (i.e. it's just pulling Requires v0.5.2 back in) or maybe there's a difference related to Pkg2 vs Pkg3 style environments — I was initially running into the bug for some code I was adding a Project.toml and trying to sort out how sub-projects should be structured... (See comments below.)

[jmert]

So it turns out I can trigger the error like in #48 by reverting #49 (but keeping the new tests) and then further substituting PyPlot in everywhere instead of JSON. (Is the difference potentially that I think JSON requires only stdlib packages whereas PyPlot needs other packages?)

[MikeInnes]

Yeah, that would likely make a difference. Would be good to find a simpler package than PyPlot that still has a dependency.

[jmert]

Switching to Colors which has a couple of dependencies doesn't trigger the error. 😕

Edit: Strike that; I only updated the submodule dependency, but it needs to be the @require-triggered package as well. That does fail now.

[MikeInnes]

👍 Thanks!