patch 8.2.2783: duplicate code for setting byte in blob, blob test may fail

Problem:    Duplicate code for setting byte in blob, blob test may fail.
Solution:   Call blob_set_append().  Test sort failure with "N".
diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 84d24cb..5eb3f3c 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -630,7 +630,7 @@
   if has('float')
     call CheckLegacyAndVim9Failure(['call sort([1.0, 0z11], "f")'], 'E975:')
   endif
-  call CheckLegacyAndVim9Failure(['call sort(["abc", 0z11], "f")'], 'E892:')
+  call CheckLegacyAndVim9Failure(['call sort([11, 0z11], "N")'], 'E974:')
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab