refactor: use more production code, fewer mocks in github tests

[ckerr]

Fifth in a series to move GitHub token + gist management into the main process.

This is a test cleanup PR:

• Adds support for app.setPath() in the Electron mock
• Throws an error if the Electron mock's app.getPath() is called with an unexpected key.
• Makes STATIC_DIR in main/constants work in both production and test environments
• In github tests, remove the mocks for
  • node:fs
  • fs-extra
  • getTemplate()
• in github tests, try to not rely on implementation details, eg not mocking/spying low-level filesystem calls. The tests shouldn't care which of fs.existsSync() or fsPromises.exists() is called by production code.

[coveralls]

coverage: 88.545% (+0.08%) from 88.468% — refactor/fewer-mocks-in-github-tests into main