SGSN_Tests: fix regression in f_send_l3_gmm_llc() which breaks TC_attach_rau_a_b

The gb_index was forgotten to given to the new function f_send_l3().
All testcases which only used the default BSSGB connection #0 continued
to work, but the TC_attach_rau_a_b is the only testcase which uses #0
and #1 at the same time.

Fixes: a05b80792266 ("sgsn: Add TC_llc_null to test if SGSN survives a LLC NULL packet")
Change-Id: Ie3dd8c613d3b3440447a282dc4545078cb927274
1 file changed