Add tests of PyLong_{As,From}{Unsigned,}Long. These are very much like

the new PyLong_{As,From}{Unsigned,}LongLong tests, so the bulk of the
code is in the new #include file testcapi_long.h, which generates
different code depending on how macros are set.  This sucks, but I couldn't
think of anything that sucked less.

UNIX headache?  If we still maintain dependencies by hand, someone who
knows what they're doing should teach whatever needs it that
_testcapimodule.c includes testcapi_long.h.