patch 9.1.0364: tests: test_vim9_builtin is a bit slow

Problem:  tests: test_vim9_builtin is a bit slow
Solution: source tests from a buffer instead of
          writing and sourcing a file (Yegappan Lakshmanan)

closes: #14614

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_winfixbuf.vim b/src/testdir/test_winfixbuf.vim
index 4ee674f..f800338 100644
--- a/src/testdir/test_winfixbuf.vim
+++ b/src/testdir/test_winfixbuf.vim
@@ -2776,11 +2776,11 @@
   call s:make_buffer_pairs()
 
   set winfixbuf
-  call assert_fails("edit something_else", "E1513")
+  call assert_fails("edit something_else", "E1513:")
 
   set nowinfixbuf
   set wfb
-  call assert_fails("edit another_place", "E1513")
+  call assert_fails("edit another_place", "E1513:")
 
   set nowfb
   edit last_place