add PositiveIntegrators.jl to downstream tests

[ranocha]

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

@ChrisRackauckas As discussed at JuliaCon yesterday, I added the package https://github.com/SKopecz/PositiveIntegrators.jl to the downstream tests of OrdinaryDiffEq.jl to ensure that we stay compatible with internal changes done here.

[ranocha]

Only the codecov part of the new downstream test failed

[ChrisRackauckas]

Interesting, codecov should be disabled. I'll have to look into that.