patch 9.0.0323: using common name in tests leads to flaky tests

Problem:    Using common name in tests leads to flaky tests.
Solution:   Rename files and directories to be more specific.
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index 7b270ff..52d41db 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -489,9 +489,9 @@
 
   if has('unix')
     " Redirecting to a directory name
-    call mkdir('Xdir')
-    call assert_fails('redir > Xdir', 'E17:')
-    call delete('Xdir', 'd')
+    call mkdir('Xredir')
+    call assert_fails('redir > Xredir', 'E17:')
+    call delete('Xredir', 'd')
   endif
 
   " Test for redirecting to a register