src/should.py: use temporary files to handle large outputs

Closes #19.
3 jobs for bug/large-output in 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Unit
passed #208445
unit

00:00:02

 
  Demo
passed #208446
demo

00:00:03

 
  Coverage
passed #208447
coverage

00:00:07

85.0%