generators: Default test template

[hilary]

Description

Update generator logic so it uses a default test template if an exercise does not have example.tt.

Motivation and Context

Most of the generators now use a consistent template. The variations that used to exist were a result of inconsistent naming and/or putting logic in views. Providing a fallback default template should encourage generator authors not to put logic in the templates.

I chose this out of the open generator improvement issues (#485) as I'm trying to reduce the complexity of writing a new generator.

How Has This Been Tested?

New functionality has unit tests
I also manually conducted integration tests

• removed example.tt from an exercise, regenerated the test file; no changes
• regenerated the test file for an exercise with example.tt; no changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

References and Closures

#485

Checklist:

• My change requires a change to the documentation.
• My change relies on a pending issue/pull request
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.