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_assert.vim b/src/testdir/test_assert.vim
index a3b45a1..1a18799 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -341,8 +341,8 @@
   call test_override('char_avail', 1)
   eval 1->test_override('redraw')
   call test_override('ALL', 0)
-  call assert_fails("call test_override('xxx', 1)", 'E475')
-  call assert_fails("call test_override('redraw', 'yes')", 'E474')
+  call assert_fails("call test_override('xxx', 1)", 'E475:')
+  call assert_fails("call test_override('redraw', 'yes')", 'E474:')
 endfunc
 
 func Test_mouse_position()