Lower waiting time for USB installation in the test suite

We currently wait one hour:

  @screen.wait('USBInstallationComplete.png', 60*60)

… which is just the same as our Jenkins inactivity timeout, so in practice when Tails Installer fails and displays an error message, instead of reporting that the job failed (which is the point of the exercise) we abort the job due to this timeout (which communicates waaaay less clearly to me, with my Tails Installer developer hat, that there’s probably a bug I should fix, as opposed to “here’s a transient failure caused by instabilities of our Jenkins setup”). My understanding is that this is what prevented the Tails Installer bug described on #10717 (closed) from being identified earlier, and I don’t want to see that happen again.

IIRC this 1h timeout was relevant for running with nested virtualization on slightly old (ThinkPad X200) hardware. I don’t think anyone does that anymore, and I would bet that on such hardware, lots of other bits of our test suite will time out anyway.

So I propose we decrease this to something like 30 minutes, i.e.:

  @screen.wait('USBInstallationComplete.png', 30*60)

If any of the watchers tells me to go ahead, and nobody raises concerns, I’ll just do it on stable, devel and feature/jessie.

Parent Task: #10288

Original created by @intrigeri on 10718 (Redmine)