Fix failing tests on rbx

The failures were due to Rbx recovering more source than expected (compared to MRI)

For example, the following fails in MRI:
module M
end

module N
  include M
end

show-source N

but on rbx it succeeds and shows the source of N

How does rbx do it? It appears to create a secret method called __module_init__ on a module whenever you include another module.
This method has its source_location set to the `include` line.