Add unit test for all engines

[Chafid]

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)
• [ ] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[willwade]

NB: SAPI Is broken right now.. https://github.com/willwade/tts-wrapper/issues/24 so this will fail

[Chafid]

Got it Will, I'll update it like in the tests/test_tts_engines.py

[willwade]

Just holding back until we can fully check things as this Im pretty sure will break the CI stuff.. Will do this later. Thanks @Chafid

[willwade]

@Chafid im going to merge this unless you can see a reason not to

[willwade]

Im going to close this - its now all in #37