TestEnv doesn't play well with 1.9's weak dependencies

[haberdashPI]

I've noticed that when using TestEnv, package extensions (a new feature in 1.9) do not get loaded when a weak dependency is loaded. (Works fine when I run tests normally).

I assume that this is a feature that hasn't been implemented yet in TestEnv.

[oxinabox]

Yep, we will need to identify and port over that code.

[oxinabox]

Looks like #53 was incorrect and we will need to split those again.

[oxinabox]

I can't see anything obvious in https://github.com/JuliaLang/Pkg.jl/pull/3264/files#diff-5139153dfc15595c21d138e94a8641dd57bd6863cb2fdcc52a1d90a715da5ee8
I suspect there is another follow up PR that made all weakdeps into strong deps for testing

[haberdashPI]

I noticed the problem I was running into was actually because I called TestEnv.activate() after using MyPackage. If I do it in the other order the extensions do get loaded. 😬 Sorry for the noise. (Perhaps there is another issue once they load, I haven't checked).