avutil/hmac: fix API/ABI compatibility with the fork

The test code is updated with some cosmetics to avoid the loop
using undefined AVHMACType values.
The old enum values will be removed in the next major bump, effectively
making both projects synced and without API or ABI issues.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
