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/configure.ac b/openbsc/configure.ac
index f330c99..90e3654 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -4,6 +4,7 @@
 	[openbsc-devel@lists.openbsc.org])
 
 AM_INIT_AUTOMAKE([dist-bzip2])
+AC_CONFIG_TESTDIR(tests)
 
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])