Resolve platform specific runtime executable
Resolve platform specific runtime executable

The path to the runtime executable depends on whether the tests are run
on mono, or on {unix, windows} corerun.

sean-gilliam/msbuild,rainersigwald/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,mono/msbuild,Microsoft/msbuild,jeffkl/msbuild,rainersigwald/msbuild,chipitsine/msbuild,mono/msbuild,chipitsine/msbuild,chipitsine/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,Microsoft/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,mono/msbuild,Microsoft/msbuild,chipitsine/msbuild,mono/msbuild,rainersigwald/msbuild,jeffkl/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild