core/conv: Add regression testing code

It tests both a given test vector encoding/decoding, and
some random coding/decoding cycles.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 5ff82e1..7a1147b 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,5 +1,5 @@
 if ENABLE_TESTS
-SUBDIRS = timer sms ussd smscb bits a5
+SUBDIRS = timer sms ussd smscb bits a5 conv
 if ENABLE_MSGFILE
 SUBDIRS += msgfile
 endif