patch 8.2.1593: tests do not check the error number properly

Problem:    Tests do not check the error number properly.0
Solution:   Add a colon after the error number. (closes #6869)
diff --git a/src/testdir/test_signs.vim b/src/testdir/test_signs.vim
index 3ee2b49..738eebf 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -1994,7 +1994,7 @@
   call assert_fails('call sign_unplacelist([[]])', "E715:")
   call assert_fails('call sign_unplacelist(["abc"])', "E715:")
   call assert_fails('call sign_unplacelist([100])', "E715:")
-  call assert_fails("call sign_unplacelist([{'id' : -1}])", 'E474')
+  call assert_fails("call sign_unplacelist([{'id' : -1}])", 'E474:')
 
   call assert_equal([0, 0, 0, 0],
 	      \ sign_undefine(['sign1', 'sign2', 'sign3', 'sign4']))