channel_test: test nr of subslots for dyn pchan, with error

Add test_dyn_ts_subslots() and call from main(). Update channel_test.ok.

This includes erratic assert to show a bug for TCH/F_PDCH in PDCH mode: the nr
of subslots should be the same as for a normal PDCH, i.e. zero. This will be
adjusted along with the fix in an upcoming commit.

Change-Id: I09685be3fb3ed1ead4577b772a9fbc31967980d1
diff --git a/openbsc/tests/channel/channel_test.ok b/openbsc/tests/channel/channel_test.ok
index 7976aee..33c8193 100644
--- a/openbsc/tests/channel/channel_test.ok
+++ b/openbsc/tests/channel/channel_test.ok
@@ -1,2 +1,3 @@
 Testing the gsm_subscriber chan logic
 Reached, didn't crash, test passed
+Testing subslot numbers for pchan types