tests/eas/acceptance: Fix test_first_cpu for OffloadMigration
tests/eas/acceptance: Fix test_first_cpu for OffloadMigration

Only the "early starter" tasks can be expected to start on a big CPU.

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