vim-patch:8.2.2949: tests failing because no error for float to string conversion

Problem:    Tests failing because there is no error for float to string
            conversion.
Solution:   Change the check for failure to check for correct result.  Make
            some conversions strict in Vim9 script.

3cfa5b16b0

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

test/old/testdir/vim9.vim

@@ -2,6 +2,10 @@
 " Use a different file name for each run.
 let s:sequence = 1
 
+func CheckDefExecFailure(lines, error, lnum = -3)
+  return
+endfunc
+
 func CheckScriptFailure(lines, error, lnum = -3)
   if get(a:lines, 0, '') ==# 'vim9script'
     return