patch 9.0.1005: a failed test may leave a swap file behind

Problem:    A failed test may leave a swap file behind.
Solution:   Delete the swap file to avoid another test to fail.  Use another
            file name.
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 73573ff..8c56730 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -455,7 +455,7 @@
 endfunc
 
 func Test_ownsyntax()
-  new Xfoo
+  new XfooOwnSyntax
   call setline(1, '#define FOO')
   syntax on
   set filetype=c