tests: subsys: llext: filter by ram/rom size

The LLEXT test suites are quite large, and should not be built
for "small" SoCs. Filter the tests based on RAM and ROM/flash
size. The following test sizes built for nrf52840 where used
to determine min sizes:

Name:                       Flash:   RAM:
llext.loader_build          105      80
llext.readonly              105      79
llext.readonly_mpu          140      95
llext.readonly_mmu          106      145
llext.writable              123      125
llext.writable_relocatable  117      129
llext.writable_slid_linking 108	     125

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>