tests: arm: Increase main stack size with no optimizations

If the test is run with the config NO_OPTIMIZATIONS enabled then the
stack size usage increases by around 80% for ARM platforms.
Increase the stack size used in test cases that enables building with no
optimizations for ARM.

Update description on TEST_ARM_CORTEX_M since it was outdated and said
it was only used for a single purpose.

Fixes: #47930
Fixes: #47929
Fixes: #47855

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>