gbproxy: Test and fix IMSI/TMSI matching

This adds a test for gbprox_set_patch_filter() and
gbprox_check_imsi().

It also fixes the masking of the type field when IMSIs are checked by
using GSM_MI_TYPE_MASK (0x07) instead of 0x0f.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index 4553648..c893ff3 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -1479,6 +1479,8 @@
   NSEI 8192, BVCI 4098, not blocked, RAI 112-332-16464-96
     NSEI mismatch                   : 1
     TLLI-Cache: 0
+=== Test IMSI/TMSI matching ===
+
 === test_gbproxy_ra_patching ===
 --- Initialise SGSN ---