s1gw: generalize f_TC_e_rab_setup_failure() for N E-RABs
The existing TC only tested PFCP Session Establishment failure with a single E-RAB. Generalize f_TC_e_rab_setup_failure() to use g_pars.erabs throughout, so it works for any num_erabs: E-RAB 0 still triggers the only PFCP Session Establishment Request (S1GW processes sequentially), while all E-RABs in the request are expected to appear in the failure response.
Add TC_e_rab_setup3_failure() which re-uses the same body with num_erabs := 3, verifying that the S1GW lists all three E-RABs as failed rather than just the first one. Also make f_tr_E_RABList() non-private so it is accessible from S1GW_Tests.ttcn.
