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_clientserver.vim b/src/testdir/test_clientserver.vim
index ab80bd2..7a75055 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -70,7 +70,7 @@
     call WaitForAssert({-> assert_equal('maybe', remote_expr(name, "testvar", "", 2))})
   endif
 
-  call assert_fails('call remote_send("XXX", ":let testvar = ''yes''\<CR>")', 'E241')
+  call assert_fails('call remote_send("XXX", ":let testvar = ''yes''\<CR>")', 'E241:')
 
   " Expression evaluated locally.
   if v:servername == ''