panic at unreachable if it's not installed with --locked

[kemingy]

Following the README instruction: cargo install sqllogictest-bin will update the tokio-postgres from 0.7.10 to 0.7.12, which has a potential breaking change that will cause a panic in

sqllogictest-rs/sqllogictest-engines/src/postgres/simple.rs

Line 46 in a6bbfdb

_ => unreachable!(),

This could be solved by cargo install sqllogictest-bin --locked, though some crates are already yanked.

[skyzh]

fixed with #233 :)