[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/channel/Makefile.am b/openbsc/tests/channel/Makefile.am
new file mode 100644
index 0000000..60defe0
--- /dev/null
+++ b/openbsc/tests/channel/Makefile.am
@@ -0,0 +1,14 @@
+INCLUDES = $(all_includes) -I$(top_srcdir)/include
+AM_CFLAGS=-Wall -ggdb3
+
+noinst_PROGRAMS = channel_test
+
+channel_test_SOURCES = channel_test.c \
+	$(top_srcdir)/src/db.c \
+	$(top_srcdir)/src/gsm_subscriber.c \
+	$(top_srcdir)/src/debug.c \
+	$(top_srcdir)/src/timer.c \
+	$(top_srcdir)/src/select.c \
+	$(top_srcdir)/src/gsm_data.c
+channel_test_LDADD = -ldl -ldbi
+