Fix fastboot parallel flash test flake

The problematic test asserts that after two async flash commands, there
are two tasks spawned. However each new flash will poll all existing
tasks once, which can cause some of them to finish. The assert can fail
if previous flash was given a perfectly aligned buffer for which it can
finish immediately.

Change-Id: I93c6fa1fc454d7f131ef8c617a63323b5c9a0f9f