test: Rename test-main.c to test-dm.c

This is the main test function for driver model but not for other tests.
Rename the file and the function so this is clear.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index fd14551..f5cc554 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -2,7 +2,7 @@
 #
 # Copyright (c) 2013 Google, Inc
 
-obj-$(CONFIG_UT_DM) += test-main.o
+obj-$(CONFIG_UT_DM) += test-dm.o
 
 # Tests for particular subsystems - when enabling driver model for a new
 # subsystem you must add sandbox tests here.