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.

src/testdir/test_eval_stuff.vim

@@ -144,7 +144,7 @@ func Test_string_concatenation()
   if has('float')
     let a = 'A'
     let b = 1.234
-    call assert_fails('echo a .. b', 'E806:')
+    call assert_equal('A1.234', a .. b)
   endif
 endfunc