shorten timespan on initdt test

[oscardssmith]

This previously was testing that you don't get a warning even though the solution was Inf. The only reason it worked at all is that Euler is simple enough that it stayed Inf rather than going to NaN and DiffEqBase incorrectly didn't consider Inf to be diverged.

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.