test/cmd/uninstall: Prevent can't modify frozen object

Fix the error:
  RuntimeError:
    can't modify frozen object
  # ./test/cmd/uninstall_spec.rb:63:in `block (3 levels) in <top (required)>'

This issue arose after merging PR https://github.com/Homebrew/brew/pull/5300.

Library/Homebrew/test/cmd/uninstall_spec.rb

@@ -60,7 +60,9 @@ describe Homebrew do
     end
 
     specify "when not developer and --ignore-dependencies is specified" do
+      described_class.args = described_class.args.dup if described_class.args.frozen?
       expect(described_class.args).to receive(:ignore_dependencies?).and_return(true)
+      described_class.args.freeze
 
       expect {
         described_class.handle_unsatisfied_dependents(opts)