iotests: 037: Use TEST_IMG override instead of "mv"

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1450752561-9300-8-git-send-email-famz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/tests/qemu-iotests/037 b/tests/qemu-iotests/037
index 9171d8c..5862451 100755
--- a/tests/qemu-iotests/037
+++ b/tests/qemu-iotests/037
@@ -51,6 +51,9 @@
 echo
 echo "== creating backing file for COW tests =="
 
+TEST_IMG_SAVE="$TEST_IMG"
+TEST_IMG="$TEST_IMG.base"
+
 _make_test_img $size
 
 function backing_io()
@@ -71,7 +74,7 @@
 
 backing_io 0 256 write | $QEMU_IO "$TEST_IMG" | _filter_qemu_io
 
-mv "$TEST_IMG" "$TEST_IMG.base"
+TEST_IMG="$TEST_IMG_SAVE"
 
 _make_test_img -b "$TEST_IMG.base" 6G