testsuite: Don't always loop in memorytexture tests

testsuite: Don't always loop in memorytexture tests

When running the tests, only run the random (and potentially large) size
download test once instead of 10 times.

There's no real benefit in doing that, both because it's unlikely to
fail only in the 2nd or 9th run and because the sizes are picked
randomly.

This also speeds up the test massively as the download test was
dominating the runtime.