Add assertion to async_com_alloc_2 for minimum img

 - Thanks to @jbmaggard for pointing this out in #435

Author: zbeekman

src/tests/unit/init_register/async_comp_alloc_2.f90

@@ -11,6 +11,8 @@ program async_comp_alloc_2
 
   sync all
 
+  if ( num_images() < 2 ) error stop "Test async_comp_alloc_2 requires at least 2 images to run"
+
   associate(me => this_image())
     if (me == 2) then
       allocate(parent_obj%arr(3))