arm: cortex_m: enable null-pointer exception detection in the tests

Enable the null-pointer dereferencing detection by default
throughout the test-suite. Explicitly disable this for the
gen_isr_table test which needs to perform vector table reads.
Disable null-pointer exception detection on qemu_cortex_m3
board, as DWT it is not emulated by QEMU on this platform.
Additionally, disable null-pointer exception detection on
mps2_an521 (QEMU target), as DWT is not present and the MPU
based solution won't work, since the target does not have
the area 0x0 - 0x400 mapped, but the QEMU still permits
read access.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>