Commit 2b0180c37fa6ffc48ee40caa831ca398b828e680 attempted to do this but
only hit one of many BN_mod_exp codepaths. Fix remaining variants and
add a test for each method.
Thanks to Hanno Boeck for reporting this issue.
(Imported from upstream's 44e4f5b04b43054571e278381662cebd3f3555e6.)
Change-Id: Ic691b354101c3e9c3565300836fb6d55c6f253ba
Reviewed-on: https://boringssl-review.googlesource.com/6820
Reviewed-by: Adam Langley <agl@google.com>