This also fixes TestGetUint to actually test CBS_get_last_u8's behavior.
Right now it can't distinguish CBS_get_last_u8 and CBS_get_u8.
BUG=129
Change-Id: Ie431bb1a828f1c6877938ba7e75c82305b54cf13
Reviewed-on: https://boringssl-review.googlesource.com/15007
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>