exercise util should use the same template

[coriolinus]

When using exercise generate, the generated file is different depending on whether or not canonical data exists for that exercise:

• if an exercise has canonical data, the tests module has a nice explanatory doccomment
• if an exercise does not have canonical data, the tests module does not have the module doccomment

Both cases should use the same basic template, with the doccomment.

[dvermd]

I have a PR for this one.
There seems to be some redundancy in the template as in [utility][utility] and [canonical data][canonical data].
Should I had another commit to dedup these ?