patch 9.0.0363: common names in test files causes tests to be flaky

Problem:    Common names in test files causes tests to be flaky.
Solution:   Use more specific names.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index c916b1a..92cf8bf 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -1235,8 +1235,8 @@
 
 " Test for creating views with manual folds
 func Test_mkview_manual_fold()
-  call writefile(range(1,10), 'Xfile')
-  new Xfile
+  call writefile(range(1,10), 'Xmkvfile')
+  new Xmkvfile
   " create recursive folds
   5,6fold
   4,7fold
@@ -1259,7 +1259,7 @@
   source Xview
   call assert_equal([-1, -1, -1, -1, -1, -1], [foldclosed(3), foldclosed(4),
         \ foldclosed(5), foldclosed(6), foldclosed(7), foldclosed(8)])
-  call delete('Xfile')
+  call delete('Xmkvfile')
   call delete('Xview')
   bw!
 endfunc