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_lambda.vim b/src/testdir/test_lambda.vim
index b43ef52..aa73462 100644
--- a/src/testdir/test_lambda.vim
+++ b/src/testdir/test_lambda.vim
@@ -308,7 +308,7 @@
 
 func Test_lambda_error()
   " This was causing a crash
-  call assert_fails('ec{@{->{d->()()', 'E15')
+  call assert_fails('ec{@{->{d->()()', 'E15:')
 endfunc
 
 func Test_closure_error()