Theme switcher e2e test flakiness

Created by: felixfbecker

In this build, the theme switcher e2e test failed: https://buildkite.com/sourcegraph/sourcegraph/builds/43747#b2fd9347-df48-4653-91a7-58d22131fa10

It failed because while the body already had the theme class (ensured by waitForSelector(), the precondition of having a theme-dark or theme-light class was not met (yet) and the assertion right after failed.

The problem here is the general problem of instantly asserting instead of using waitForFunction()/waitForSelector() or retry().