Expand dev-cmd/livecheck tests

Library/Homebrew/test/dev-cmd/livecheck_spec.rb

@@ -21,4 +21,11 @@ describe "brew livecheck", :integration_test do
       .and not_to_output.to_stderr
       .and be_a_success
   end
+
+  it "gives an error when no arguments are given and there's no watchlist" do
+    expect { brew "livecheck" }
+      .to output(/Invalid usage: A watchlist file is required when no arguments are given\./).to_stderr
+      .and not_to_output.to_stdout
+      .and be_a_failure
+  end
 end