testsuite: Make memorytexture tests random

testsuite: Make memorytexture tests random

Instead of picking a few numbers in advance and running them through the
test gauntlet every time, pick the random numbers at runtime.

This both increases the test coverage in that it ultimately tests more
combinations across many runs and it reduces the runtime of individual
runs because every tun only runs the download tests twice (with 1px and
the random size) instead of 5 times.

And that speedup benefits the CI, where the asan runs would cause this
test to timeout sometimes.