patch 8.1.1660: assert_fails() does not fail inside try/catch

Problem:    Assert_fails() does not fail inside try/catch.
Solution:   Set trylevel to zero. (Ozaki Kiichi, closes #4639)
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 0d59a43..900710b 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -166,6 +166,12 @@
   call remove(v:errors, 0)
 endfunc
 
+func Test_assert_fails_in_try_block()
+  try
+    call assert_equal(0, assert_fails('throw "error"'))
+  endtry
+endfunc
+
 func Test_assert_beeps()
   new
   call assert_equal(0, assert_beeps('normal h'))