Replace Karma test runner

[Acconut]

The Karma test runner is deprecated and no longer maintained: https://github.com/karma-runner/karma#karma-is-deprecated-and-is-not-accepting-new-features-or-general-bug-fixes

Recommended replacements are:

• Web Test Runner, which also provides an integration with BrowserStack and Jasmine
• jasmine-browser-runner, which is developed for Jasmine and also supports running on jasmine-browser-runner

The BrowserStack tests are currently failing very often. Hopefully such a replacement could make them more stable.