tests/eas/hmp_parity: don't hardcode the number of tasks in WakeMigration
tests/eas/hmp_parity: don't hardcode the number of tasks in WakeMigration

WakeMigration creates two tasks, which is ok if the test system has two
or more big cpus, but it will fail with one big cpu.  We don't need to
hardcode the number of tasks, we can generalize the test by creating as
many as there are big cpus.

bjackman/lisa,mdigiorgio/lisa,credp/lisa,ARM-software/lisa,arnoldlu/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,mdigiorgio/lisa,bjackman/lisa,joelagnel/lisa,credp/lisa,ARM-software/lisa,arnoldlu/lisa,credp/lisa,joelagnel/lisa