iotests: Use _rm_test_img for deleting test images

Just rm will not delete external data files.  Use _rm_test_img every
time we delete a test image.

(In the process, clean up the indentation of every _cleanup() this patch
touches.)

((Also, use quotes consistently.  I am happy to see unquoted instances
like "rm -rf $TEST_DIR/..." go.))

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-16-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/tests/qemu-iotests/050 b/tests/qemu-iotests/050
index 272ecab..cdc5356 100755
--- a/tests/qemu-iotests/050
+++ b/tests/qemu-iotests/050
@@ -29,8 +29,8 @@
 _cleanup()
 {
     _cleanup_test_img
-    rm -f "$TEST_IMG.old"
-    rm -f "$TEST_IMG.new"
+    _rm_test_img "$TEST_IMG.old"
+    _rm_test_img "$TEST_IMG.new"
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15