patch 8.2.0261: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5645)

src/testdir/test_plus_arg_edit.vim

@@ -32,3 +32,16 @@ func Test_edit_bad()
   bw!
   call delete('Xfile')
 endfunc
+
+" Test for ++bin and ++nobin arguments
+func Test_binary_arg()
+  new
+  edit ++bin Xfile1
+  call assert_equal(1, &binary)
+  edit ++nobin Xfile2
+  call assert_equal(0, &binary)
+  call assert_fails('edit ++binabc Xfile3', 'E474:')
+  close!
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab