test-modem-helpers: fix warnings with -Wswitch-enum

test-modem-helpers.c: In function ‘test_cgev_indication’:
  test-modem-helpers.c:2165:9: error: enumeration value ‘MM_3GPP_CGEV_UNKNOWN’ not handled in switch [-Werror=switch-enum]
   2165 |         switch (type) {
        |         ^~~~~~
  cc1: all warnings being treated as errors

src/tests/test-modem-helpers.c

@@ -2163,6 +2163,7 @@ test_cgev_indication (const CgevIndicationTest *t)
         g_debug ("[%u] type: %u", i, type);
 
         switch (type) {
+        case MM_3GPP_CGEV_UNKNOWN:
         case MM_3GPP_CGEV_NW_DETACH:
         case MM_3GPP_CGEV_ME_DETACH:
             break;