Add e2e testing for templates

[jmetz]

Is your feature request related to a problem? Please describe.
This is related to the development cycle for cargo-mobile2, and in particular adding templates. In a recent PR (#406 + #408) ,
I had accidentally not committed a final missing semi-colon from a handlebar template file; at the moment, the
developer / contributor workflow relies on contributors catching handlebars errors themselves by performing
essentially manual end-to-end testing.

Describe the solution you'd like
Instead if a system can be implemented that can effectively discover and test all templates using default entries, this would go
some way to improving the development / contribution process.

Describe alternatives you've considered
Sticking with the current manual testing 👎