tests: Add a test stub for the subchan_demux tests

Tobias has worked on CSD support and Dieter has helped with a CSD
trau fix. This is a stub that should allow to easily write a test
case for the syncing issue observed by Tobias.

Run the tests:
$ make check

In case of failure:
$ vi tests/testsuite.dir/1/testsuite.log

Add the actual content:
$ edit tests/subchan_demux/subchan_demux_test.c

Update the test result:
$ ./tests/subchan_demux/subchan_demux_test > tests/subchan_demux/subchan_demux_test.ok
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 724285c..8704014 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -6,7 +6,8 @@
 		  e1inp_ipa_bts_test	\
 		  e1inp_hsl_bsc_test	\
 		  e1inp_hsl_bts_test	\
-		  ipa_proxy_test
+		  ipa_proxy_test	\
+		  subchan_demux/subchan_demux_test
 
 e1inp_ipa_bsc_test_SOURCES = e1inp_ipa_bsc_test.c
 e1inp_ipa_bsc_test_LDADD = $(top_builddir)/src/libosmoabis.la \
@@ -29,6 +30,10 @@
 			$(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) \
 			$(LIBOSMOVTY_LIBS)
 
+subchan_demux_subchan_demux_test_SOURCES = subchan_demux/subchan_demux_test.c
+subchan_demux_subchan_demux_test_LDADD = $(top_builddir)/src/libosmoabis.la \
+			$(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) \
+			$(LIBOSMOVTY_LIBS)
 
 # boilerplate for the tests
 # The `:;' works around a Bash 3.2 bug when the output is not writeable.
@@ -49,7 +54,8 @@
                echo '  [$(PACKAGE_URL)])'; \
              } >'$(srcdir)/package.m4'
 
-EXTRA_DIST = testsuite.at $(srcdir)/package.m4 $(TESTSUITE)
+EXTRA_DIST = testsuite.at $(srcdir)/package.m4 $(TESTSUITE) \
+	subchan_demux/subchan_demux_test.ok
 
 TESTSUITE = $(srcdir)/testsuite