Changes

s1ap_proxy_test: do more s1ap_proxy:fetch_erab/2

Just like we do in test_e_rab_setup[_dup], let's test fetching
E-RAB info by UID in test_initial_context_setup[_dup] for the
sake of completeness and consistency.

Change-Id: I8e9ffc22be3bc352a8d3022b1c7b0c50699ff493