bn_test's output is meant to be piped to bc, but this got broken somewhat:

- OpenSSL uses uppercase hex rather than BoringSSL's lowercase. bc only accepts
  uppercase. Document that this needs some shell pipeline until we replace
  them with better tests because this is all ridiculous.

- Some stderr outputs moved to stdout to avoid cluttering stdout. Just remove
  them. The operations are fast enough to not need progress.

- To cut down on noise, only write the bc transcript given a command-line flag.
  Also remove the -results flag since it's pointless. (It writes only the
  results and not the inputs.)

Change-Id: I08f87cac1e03fab461f0dc40b9d4285bd877807d
Reviewed-on: https://boringssl-review.googlesource.com/4896
Reviewed-by: Adam Langley <agl@google.com>