patch 8.2.2873: not enough tests for writing buffers

Problem:    Not enough tests for writing buffers.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #8229)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index f675697..96cf16c 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2630,6 +2630,7 @@
 func Test_glob()
   call assert_equal('', glob(test_null_string()))
   call assert_equal('', globpath(test_null_string(), test_null_string()))
+  call assert_fails("let x = globpath(&rtp, 'syntax/c.vim', [])", 'E745:')
 
   call writefile([], 'Xglob1')
   call writefile([], 'XGLOB2')