Fix and re-enable Thermal tests

[ven-k]

Checklist

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

Additional context

Along with related test fixes, Motor and Piston are split out into dedicated files as these can serve as demos.