[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/tests/Makefile.am b/openbsc/tests/Makefile.am
index 2b72c9c..2d4e81c 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -1 +1 @@
-SUBDIRS = debug timer sms gsm0408 db
+SUBDIRS = debug timer sms gsm0408 db channel