Add test for updateMobileRequiresDun()

This test validates that when a mobile upstream has been requested
changing the "DUN required" state results in updating the requested
network appropriately.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-test passes
Bug: 32163131

Change-Id: Ia3a1f6493ea348eb75e334344468a980206f618f