Convert bn_test to C++.

Along the way, fix a host of missing failure checks. This will save some
headache when it comes time to run these under the malloc failure tests.

Change-Id: I3fd589bd094178723398e793d6bc578884e99b67
Reviewed-on: https://boringssl-review.googlesource.com/4126
Reviewed-by: Adam Langley <agl@google.com>
5 files changed