The plugin modules in these tests get statically linked with a separate
copy of GLib so they end up calling vfuncs in their own copy of GLib.
Fixes#1648
On Visual Studio, Meson builds modules as xxxx.dll, not libxxxx.dll when
xxxx is specified as the name for the shared_module() build directive.
This means that in the test programs if we expect for libxxxx for the
module name, the test will fail as there is no libxxxx.dll but there is
xxxx.dll. This makes the test program look for the module files
correctly.
The file extension of the GIO module could be something other than .so,
depending on platform. Use G_MODULE_SUFFIX so that the test will run
correctly on non-*nix platforms, such as Windows.
https://bugzilla.gnome.org/show_bug.cgi?id=719344
