Reinstate CI tests with Visual Studio 2015

[squeek502]

Follow-up from https://github.com/luvit/luv/pull/748, and more specifically https://github.com/luvit/luv/commit/549599faffeb0d43b100ce850747a863294a27b4 which removed running the Visual Studio 2015 image from our CI

The point [of reinstating it] would be to ensure we match the Libuv supported platforms, which still lists >= MSVC 2015. That is, even though Windows >= 10 is now required, MSVC >= 2015 is still documented as supported.

So, the goal would just be to test to ensure that we can always still build with MSVC 2015.

• Using the Appveyor image, but building against Libuv 1.49.2, would get us partway there, but any >= 1.50.0 specific code would not be tested
• None of the GitHub runner images seem to have MSVC 2015 installed AFAICT

So, for a full solution, we'd likely have to deal with setting up MSVC 2015 on whatever runner we go with.