Implement more end-to-end tests

[ItsSim]

End-to-end testing was recently introduced into the project.

Tests are run using Playwright, an automated testing framework created by Microsoft that supports testing Electron applications.

Check out the end-to-end testing guide to learn how to create tests for the launcher. Basically: pick a feature, and create a test that performs the same actions a user would, with an expected successful or failure outcome.

By creating tests, we can ensure that every critical part of the launcher works as expected and any breaking changes are quickly identified before a release is sent out to end users.

There is currently one test file created for the installer feature of the launcher in src/tests/e2e/installer.spec.js.