tests/lint: add test for prototype conversions in C90

The purpose of warning 259 is to find function calls that differ in the
ABI.  The warning's original purpose was not to warn about lossy
conversions, that's just a side effect.

Warning 259 had been implemented before C99 was published, which is more
than 20 years ago.  In the meantime, almost all code has migrated to
using function prototypes.  With the default lint flags from NetBSD's
<sys.mk>, it would rather make sense to focus on lossy conversions now.

To prepare for potentially upcoming differences in lint's C90 and C99
modes, clone the test now as far as possible.  The test for C90 mode is
smaller than for C99 mode, since 'long long' was not available back
then.

(rillig)