src/should.py: remove a spurious 'failed:0' with --xml

Closes #42.
3 jobs for feature/42-remove-spurious-failed-0 in 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Unit
passed #296808
unit

00:00:01

 
  Demo
passed #296809
demo

00:00:04

 
  Coverage
passed #296810
coverage

00:00:06

86.0%