tests: share stubs.h from msc_vlr_test as stubs.c

We also need stubs for the upcoming db_sms tests.

Due to a known bug of automake [1], we cannot use 'subdir-objects',
so as a side effect this change introduces some autoreconf warnings.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=752993

Change-Id: I8846c940f2695fd33e1007fecac83e73f508bb34
diff --git a/tests/msc_vlr/msc_vlr_test_rest.c b/tests/msc_vlr/msc_vlr_test_rest.c
index 620652c..1b998ad 100644
--- a/tests/msc_vlr/msc_vlr_test_rest.c
+++ b/tests/msc_vlr/msc_vlr_test_rest.c
@@ -22,7 +22,7 @@
  */
 
 #include "msc_vlr_tests.h"
-#include "stubs.h"
+
 #include <osmocom/msc/vlr.h>
 
 #if 0