Test suite: use 7200 MiB virtual USB drives when we really mean 8 GiB (refs: #12707).

In the real world, USB sticks labeled "8 GB" can be much smaller,
so Tails Installer will accept anything that's at least 7200 MiB.

This commit makes us exercise something closer to what happens in the real
world, and incidentally it'll save storage space on our isotesters
and improve test suite performance a bit :)