patch 9.0.0336: tests are flaky because of using a common file name

Problem:    Tests are flaky because of using a common file name.
Solution:   Rename files and directories to be more unique.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 3c9fbec..8cb5060 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ