ct_opts allows sys_config to point to a non exisiting file

I believe this is a silent failure - if you specify a non-existing file in `ct_opts` as a `sys_config` no error will be thrown. Discovered it because of a typo in the filename. I think that `rebar3` could provide a guard for that.

Eager to submit a PR if you believe that it would be an enhancement.

Rebar3 version: 3.24.0