Cleanups and fixes for Visual Studio builds

Hi,

This makes some cleanups for the Visual Studio builds in the Meson build files, as well as dropping a g_autoptr() in the test programs:

• Drop the msvc subdir, since we do not need its wrapper math.h anymore for 5.x, since we already require a C99 compiler to build 5.x, where the stock math.h should contain everything we need. Clean up the include directories in gtksourceview/meson.build as a result.
• Drop the core_dep from the dependencies of the programs in test, since we should already have decent support for Meson's whole_archive: directive for Visual Studio.
• Drop a g_autoptr() usage from the test programs.

With blessings, thank you!