test: fix errstr matching for musl libc
test: fix errstr matching for musl libc

This makes the tests pass on musl[1] based distros like Alpine Linux.

[1]: https://www.musl-libc.org/

jinguoli/hiredis,redis/hiredis,jinguoli/hiredis,redis/hiredis,jinguoli/hiredis,redis/hiredis