Don't check for breakdown if adaptive=false

[DanielVandH]

This changes phiv_timestep to no longer check for a happy-breakdown if adaptive=false.

Checklist

• Appropriate tests were added

I'm not sure what example to come up with to get a new direct test for this. The only example I have is from private code.

• 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

[DanielVandH]

Actually this seems to lead to a regression in #143 since, without checking for breakdown, that example takes an extremely long time since the default is adaptive=false. Is there a better solution for this? Maybe checking that adaptive=true and simultaneously tau wasn't specified?