Fix frozen string literal warning in Mockery

Thanks to @radville for pointing this out in #669. I've tackled it
slightly differently by using an Array to build up the lines and then
joining them into a String at the end rather than calling `String#dup`
multiple times.

Previously I was seeing a lot of the following warnings when running
the tests under Ruby v3.4:

    lib/mocha/mockery.rb:118: warning: literal string will be frozen in the future

lib/mocha/mockery.rb

@@ -114,11 +114,11 @@ def sequences
     end
 
     def mocha_inspect
-      message = ''
-      message << "unsatisfied expectations:\n- #{unsatisfied_expectations.map(&:mocha_inspect).join("\n- ")}\n" if unsatisfied_expectations.any?
-      message << "satisfied expectations:\n- #{satisfied_expectations.map(&:mocha_inspect).join("\n- ")}\n" if satisfied_expectations.any?
-      message << "states:\n- #{state_machines.map(&:mocha_inspect).join("\n- ")}\n" if state_machines.any?
-      message
+      lines = []
+      lines << "unsatisfied expectations:\n- #{unsatisfied_expectations.map(&:mocha_inspect).join("\n- ")}\n" if unsatisfied_expectations.any?
+      lines << "satisfied expectations:\n- #{satisfied_expectations.map(&:mocha_inspect).join("\n- ")}\n" if satisfied_expectations.any?
+      lines << "states:\n- #{state_machines.map(&:mocha_inspect).join("\n- ")}\n" if state_machines.any?
+      lines.join
     end
 
     def on_stubbing(object, method)