tests: build_all: modem: set minimum flash requirement

Based on building for the frdm_k64f and mimxrt1050_evk, the
drivers.modem.build build seems to require at least 115KB of flash. Set
this flash limit so that the test will be skipped for devices with less
flash available.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>