bsim: Bluetooth: Mesh: refactor suspend test

Renames tx to DUT and rx to Tester to clarify roles in test.

Re-uses the same body for suspend/resume and suspend/disable/resume
for DUT with a parameter to toggle disabling BT.

Re-uses the same Tester config in both existing test-cases as the
previous configs were duplicates.

Updates the dut suspension status in the message handler.

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