Add tests for calls between Fiddle and FFI
Add tests for calls between Fiddle and FFI

This is particular interesting because FFI stores thread local call info
on the stack, which is retrieved when a callback is received.
This is something that Fiddle doesn't do, but a callback should be handled
gracefully nevertheless.

The test "from fiddle to ffi" fails without the previous commit.

These tests succeed on MRI and on JRuby (provided
https://github.com/jruby/jruby/pull/4518 will be merged).

ffi/ffi,ffi/ffi,ffi/ffi