add skeleton testing for the MULTICALL macros

add skeleton testing for the MULTICALL macros

The macros dMULTICALL, PUSH_MULTICALL, MULTICALL and POP_MULTICALL
are completely untested in core apart from incidentally in List-Util.
The exercise they get there is probably quite comprehensive, but it's
not explicitly testing the macros themselves.

Add a hook and new test file to XS::APItest specifically for this purpose.
Currently the test file is almost empty.

The multicall_each function is shamelessly stolen from List:;Util::first.