Improve test coverage

The goal is test all the possible scenarios possible for a template, and test whatever can be tested. There are parts of the codebase that can't be tested because this is a plugin and can't exist without the joplin desktop application. But most parts can be tested by mocking the behaviour of Joplin APIs.

Current coverage status - https://coverage.joplin-templates.nishantwrp.com

Pull requests adding new tests or enhancing existing tests are greatly appreciated. Please feel free to post any queries here or get in touch with me [directly](https://twitter.com/nishantwrp).