patch 8.0.0254: error message of assert functions is sometimes incomplete

Problem:    When using an assert function one can either specify a message or
            get a message about what failed, not both.
Solution:   Concatenate the error with the message.
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 8c54bdd..986f0d9 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -121,6 +121,12 @@
   call assert_fails('call assert_inrange(1, 1)', 'E119:')
 endfunc
 
+func Test_assert_with_msg()
+  call assert_equal('foo', 'bar', 'testing')
+  call assert_match("testing: Expected 'foo' but got 'bar'", v:errors[0])
+  call remove(v:errors, 0)
+endfunc
+
 func Test_user_is_happy()
   smile
   sleep 300m