Evaluate new environment for frontend tests

We are currently using Karma in combination with Jasmine to run our frontend tests. This has a few downsides, namely:

• we are relying on the now unmaintained jasmine-jquery (https://gitlab.com/gitlab-org/gitlab-ce/issues/24068)
• tests are all running in the same context and not sandboxed (https://gitlab.com/gitlab-org/gitlab-ce/issues/30322, https://gitlab.com/gitlab-org/gitlab-ce/issues/24100, https://gitlab.com/gitlab-org/gitlab-ce/issues/44238)
• we have no snapshots (https://gitlab.com/gitlab-org/gitlab-ce/issues/32524)
• there are no built-in async assertions / Promise based tests (https://gitlab.com/gitlab-org/gitlab-ce/issues/43323)
• Jasmine is not even listed as an option in the Vue testing guidelines: https://vue-test-utils.vuejs.org/guides/#choosing-a-test-runner

Alternatives to look at

I suggest we limit this evaluation to the tools that are described in the Vue testing guidelines:

• Jest: https://vue-test-utils.vuejs.org/guides/#testing-single-file-components-with-jest
• Mocha: https://vue-test-utils.vuejs.org/guides/#testing-single-file-components-with-mocha-webpack
• Karma + Mocha: https://vue-test-utils.vuejs.org/guides/#testing-single-file-components-with-karma

Criteria to evaluate

• solves the above mentioned points
• get it running with our tests takes less than one release cycle
• running the tests does not take much longer than currently (2 minutes)
• popularity (in general but also within the GitLab team)
• supports everything that we are currently using in our tests:
  • HTML fixtures
  • mocks / spies
  • manipulating clock
  • calculating test coverage
  • assertions on the DOM
  • run individual tests