Fix coverage + add external test.

Author: ioquatix

Diff for: config/external.yaml

@@ -0,0 +1,3 @@
+lively:
+  url: https://github.com/socketry/lively
+  command: bundle exec sus

Diff for: config/sus.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2024, by Samuel Williams.
+
+$LOAD_PATH << ::File.expand_path("../ext", __dir__)
+
+require "covered/sus"
+include Covered::Sus