bsim: Bluetooth: Mesh: Add gatt suspension test

Adds two test-cases to `test_suspend.c`, both checking that gatt
advertisement is stopped when suspending Mesh, and that is started again
when Mesh is resumed.

The first test-case involves suspending and resuming Mesh, and
the second test-case involves suspending Mesh and disabling Bluetooth,
then re-enabling Bluetooth and resuming Mesh.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>