Parameter validation was previously performed and tested unconditionally
for the ChaCha/Poly modules. This commit therefore only needs go guard the
existing tests accordingly and use the appropriate test macros for parameter
validation.
This module used (len, pointer) while (pointer, len) is more common in the
rest of the library, in particular it's what's used in the CMAC API that is
very comparable to Poly1305, so switch to (pointer, len) for consistency.
