patch 9.0.0846: using assert_fails() may cause hit-enter prompt

Problem:    Using assert_fails() may cause hit-enter prompt.
Solution:   Set no_wait_return. (closes #11522)
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 89ac012..42dd6a9 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -338,6 +338,12 @@
   endtry
 endfunc
 
+func Test_assert_fails_in_timer()
+  " should not cause a hit-enter prompt, which isn't actually checked here
+  call timer_start(0, {-> assert_fails('call', 'E471:')})
+  sleep 10m
+endfunc
+
 func Test_assert_beeps()
   new
   call assert_equal(0, assert_beeps('normal h'))