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/.gitignore b/.gitignore
index e1979ee..f862166 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,6 +32,9 @@
 tests/package.m4
 tests/testsuite
 tests/testsuite.log
+tests/subchan_demux/.dirstamp
+tests/subchan_demux/subchan_demux_test
+
 
 
 # vi/vim files