tests drivers.flash.common.default: Fix allow list

qemu_x86 was listed as an integration platform but not in the
allow list causing twister to fail:

Error found:
tests/drivers/flash/common/drivers.flash.common.default
on qemu_x86 (Not in testsuite platform allow list but is one
of the integration platforms)

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>