vim-patch:9.0.0106: illegal byte regexp test doesn't fail when fix is reversed

Problem:    Illegal byte regexp test doesn't fail when fix is reversed.
Solution:   Make sure illegal bytes end up in sourced script file.

https://github.com/vim/vim/commit/cb36c2a3cdd5610487dd830d430bf022f288328d

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_regexp_utf8.vim

@@ -562,12 +562,8 @@ func Test_match_invalid_byte()
 endfunc
 
 func Test_match_illegal_byte()
-  let lines =<< trim END
-      silent! buffer 藩\c
-      next 藩
-      0scriptnames
-      source
-  END
+  " Text has illegal bytes which need to be set explicitly
+  let lines = ["norm :set no\x01\<CR>", "silent n\xff", "silent norm :b\xff\<CR>"]
   call writefile(lines, 'Xregexp')
   call system(GetVimCommand() .. ' -X -Z -e -s -S Xregexp -c qa!')