REGTEST: add sample_fetches/hashes.vtc to validate hashes

REGTEST: add sample_fetches/hashes.vtc to validate hashes

This regtest validates all hashes that we support, on all input bytes from
0x00 to 0xFF. Those supporting avalanche are tested as well. It also tests
len(), hex() and base64(). It purposely does not enable sha2() because this
one relies on OpenSSL and there's no point in validating that OpenSSL knows
how to hash, what matters is that we can test our hashing functions in all
cases. However since the tests were written, they're still present and
commented out in case that helps.

It may be backported to supported versions, possibly dropping a few algos
that were not supported (e.g. crc32c requires 1.9 minimum).

Note that this test will fail on crc32/djb2/sdbm/wt6 unless patches
"BUG/MINOR: stream: init variables when the list is empty" and
"BUG/MAJOR: hashes: fix the signedness of the hash inputs" are included.