system: rename *.conf files to ensure proper execution order

At bootstrap we want to start udev as soon as possible (after any
services registered by plugins).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

system/90-testserv.conf

@@ -0,0 +1,4 @@
+# Test service, only used for `make check`, not for public use
+service [S123456789] name:testserv pid:!/run/testserv.pid \
+	<boot/testserv> cgroup.system \
+	/sbin/serv -n -p -i testserv -- Test serv daemon