Added testing for macOS & GTK templates

[TimLariviere]

As discussed in #287, we were not testing the macOS and GTK templates in the build process.

[TimLariviere]

Travis CI seems happy with this.

Oddly enough, on my machine (macOS), when building the GTK project it deletes the dll of the shared project.
Thus the macOS project fails due to not being able to find the dll.

Does someone have a similar problem? @mallibone

[dsyme]

This is fantastic. I checked the travis log and it is indeed building macOS

�[39;49m�[39;49m�[37m  /Library/Frameworks/Mono.framework/Versions/5.18.0/lib/mono/fsharp/fsc.exe -o:obj/Debug/testapp269.macOS.exe

[mallibone]

@TimLariviere interesting I will give it a spin later today, I did not have any issues back before we were "fighting" with Travis 🙃

I am looking forward to running this PR on the Azure DevOps PR.

[mallibone]

@TimLariviere I pulled this branch and ran ./build.sh Test without any issues locally on my Mac running Mojave 10.14.2, latest XCode and Visual Studio (stable). But it does seem odd that it fails on your end.

[TimLariviere]

Well I'm glad it's only on my end.
Maybe VS for Mac 2019 is doing something funny.