tests: Introduce running tests with GNU autotest in OpenBSC

The quality of the tests is of different value but it is good to
get started and improve from here.
diff --git a/openbsc/tests/channel/channel_test.ok b/openbsc/tests/channel/channel_test.ok
new file mode 100644
index 0000000..7976aee
--- /dev/null
+++ b/openbsc/tests/channel/channel_test.ok
@@ -0,0 +1,2 @@
+Testing the gsm_subscriber chan logic
+Reached, didn't crash, test passed