Make tests/test_rhash.sh portable

I found some tests fail on some locales (I mainly use LANG=en_US.UTF-8).

The issue is, the way to generate test vector is not portable enough. For instance, on the GNU/Linux environment which LANG=en_US.UTF-8, `awk 'BEGIN{printf("%c",0x80)}'` prints UTF-8 representation of Unicode codepoint U+0080 (0xc2 0x80) instead of single byte 0x80.

I think the only way to resolve the issue is embed test vectors in the directory as this commit does (maybe you could modify locale-related environment variables but I believe this is the most portable solution).