tests: Introduce a very basic gsm0808 test

This tests verifies the content of the GSM 08.08 messages, it does
not verify the remaining headroom (which the SCCP/IPA code relies
on being plenty to prepend the header). More to come in the future.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 348355f..eff1ac4 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,5 +1,5 @@
 if ENABLE_TESTS
-SUBDIRS = timer sms ussd smscb bits a5 conv auth lapd
+SUBDIRS = timer sms ussd smscb bits a5 conv auth lapd gsm0808
 if ENABLE_MSGFILE
 SUBDIRS += msgfile
 endif