refactor: create base template & test utils

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

DRYs up the template code with a base template class (and new module @electron-forge/template-base). The template tests were also moved to their respective modules. As a result of that, a new module for test utility functions (@electron-forge/test-utils) needed to be created as well.