If installed tests aren't enabled, installed_testdir isn't set, so it winds up
installing to $(DESTDIR)/files.

diff -x config.log -x config.status -ru tmp/glib-networking-2.38.1/tls/tests/Makefile.am work/glib-networking-2.38.1/tls/tests/Makefile.am
--- tmp/glib-networking-2.38.1/tls/tests/Makefile.am	2013-10-15 14:25:27.000000000 +0100
+++ work/glib-networking-2.38.1/tls/tests/Makefile.am	2013-10-18 14:32:12.925000000 +0100
@@ -35,6 +35,8 @@
 
 endif
 
+if ENABLE_INSTALLED_TESTS
+
 testfilesdir = $(installed_testdir)/files
 testfiles_DATA =				\
 	files/ca.pem				\
@@ -54,4 +56,6 @@
 	files/server-self.pem			\
 	$(NULL)
 
+endif
+
 EXTRA_DIST += $(testfiles_DATA)
