pw_hash: add more bcrypt tests

• check for invalid salt values for all bcrypt variants
• check for valid return value on glibc 2.28

I am not entirely sure this cutoff is correct, but RHEL 8 runs glibc
2.28 and supports bcrypt, and Ubuntu 18.04 Bionic has glibc 2.27 and
does not. Ubuntu Focal 20.04 has glibc 2.31 and also supports bcrypt.
We will see what the test harness reports :-)