sgsn: Introduce tests for several cell change related scenarios

Test TC_cell_change_different_ci_data provides test case for
fix in osmo-sgsn.git Change-Id I2c14e1d65575f54212924f7c5f0a2f4c1b76ec81

Related: SYS#4909
Change-Id: I2158685bf817d4bf064bb4d2ef5aa96ca252fe21
1 file changed