Make tests faster by avoiding unnecessary waiting

[cwalther]

As I suspected yesterday, it seems like the tests can actually be sped up massively by avoiding hardcoded wait times: Puppeteer allows us to wait for the specific element we want to check instead of waiting for a fixed time and hoping it will be there by then. I’m not sure if there was a reason for doing it this way, but replacing the waits seems to work well for me.

This reduces the total testing time from 26s to 7s for me.

The commit is stacked on #78 because only that allowed me to get meaningful figures, but is independent and could be rebased if desired for a neater branching layout.

[yanick]

I’m not sure if there was a reason for doing it this way

No wise hidden motives there. If I recall correctly, I was trying different end-to-end testing frameworks (first Nightwatch, then the jest + puppeter combo), and didn't get around to optimize the tests. This speedup by a factor of 9 is... very welcome. :-)