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/tests/Makefile.am b/openbsc/tests/Makefile.am
index 273c84f..773830b 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -1,4 +1,4 @@
-SUBDIRS = gsm0408 db channel mgcp gprs abis gbproxy trau
+SUBDIRS = gsm0408 db channel mgcp gprs abis gbproxy trau subscr
 
 if BUILD_NAT
 SUBDIRS += bsc-nat bsc-nat-trie