tests: Disable 113 generatorcustom on VS backends

Can only test this by checking the compiler id, but that's good enough.
Disabling so we can get #995 in which will help keep the VS backend in
a better state w.r.t. other PRs.

I've opened #1004 to track this in the meantime.

test cases/common/113 generatorcustom/meson.build

@@ -1,5 +1,6 @@
 project('generatorcustom', 'c')
 
+if meson.get_compiler('c').get_id() != 'msvc'
   creator = find_program('gen.py')
   catter = find_program('catter.py')
 
@@ -15,3 +16,6 @@ allinone = custom_target('alltogether',
       command : [catter, '@INPUT@', '@OUTPUT@'])
 
   executable('proggie', 'main.c', allinone)
+else
+  error('MESON_SKIP_TEST: Skipping test on VS backend; see: https://github.com/mesonbuild/meson/issues/1004')
+endif