Modify ICBMSG so that it could be used in no-multithreading.

Signed-off-by: Magdalena Pastula <magdalena.pastula@nordicsemi.no>
(cherry picked from commit 5de1c09)

…4L15

Add testcases for ICBMSG on nRF54L15 with only one endpoint,
including non-multithreading case.

Signed-off-by: Magdalena Pastula <magdalena.pastula@nordicsemi.no>
(cherry picked from commit 4952076)

Add yaml configurations to enable eGPIO tests with sysbuild
and different backends.

Cherry-picked from nrfconnect#1910 PR.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>

Add testcase for eGPIO with icbmsg backend.

Signed-off-by: Magdalena Pastula <magdalena.pastula@nordicsemi.no>