chore: made sure that cargo-hack runs testcases with the --locked option

[CommanderStorm]

This PR is basically #180 (comment), but possibly less work for you to accept/reject.

Hope this helps ^^

(also includes the fix for the mongo-example from #178)

[DDtKey]

Libraries don't usually have Cargo.lock commited. Thus, it doesn't help here
It's important, because users of crate may have different versions in their lock files and we expect testcontainers to work with latest ones