Adjust NetBSD/sha2.cc to be portable to more environments

Summary:
By an accident part of the tests contained hardcoded checksums
for external files that will differ between setups.

Reviewers: mgorny

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D56104

llvm-svn: 350097