[channel] Add a test case for the request foo in gsm_subscriber.c

This is simulating a successfull request and is causing
a segfault. The test is done with a "mock object" in this
case a minimalistic paging implementation which is enough
to show the crash.

Compare that the callback data is supposed to be the same....
diff --git a/openbsc/configure.in b/openbsc/configure.in
index b886e7a..94fb15f 100644
--- a/openbsc/configure.in
+++ b/openbsc/configure.in
@@ -45,4 +45,5 @@
     tests/sms/Makefile
     tests/gsm0408/Makefile
     tests/db/Makefile
+    tests/channel/Makefile
     Makefile)