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_substitute.vim b/src/testdir/test_substitute.vim
index 7e7ccff..df01e06 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -141,7 +141,7 @@
 
 func Test_substitute_repeat()
   " This caused an invalid memory access.
-  split Xfile
+  split Xsubfile
   s/^/x
   call feedkeys("Qsc\<CR>y", 'tx')
   bwipe!
@@ -1068,7 +1068,7 @@
   new
   norm o0000000000000000000000000000000000000000000000000000
   func EditScript()
-    silent! scr! Xfile
+    silent! scr! Xsedfile
   endfunc
   s/\%')/\=EditScript()