tests: twister_blackbox: fix vendor filter test

Commit 6dc27a4 ("boards: align vendor entry in board.yml files")
unknowingly broke Twister blackbox tests by correctly placing Atom
targets under the 'intel' vendor. Since this was not previously the
case, the Twister blackbox vendor filter test for 'intel' was actually
expecting 'qemu_x86/atom' to be filtered out as "Not a selected vendor
platform."

This commit updates test_filter.py with the new vendor flags.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>