bsc/test: Add tests for gsm_subscriber base

This commit adds test for the generic part of gsm_subscriber like
reference counting and flag usage.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 5e705c7..69be8b6 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -192,6 +192,7 @@
     tests/smpp/Makefile
     tests/trau/Makefile
     tests/sgsn/Makefile
+    tests/subscr/Makefile
     doc/Makefile
     doc/examples/Makefile
     Makefile)