Use classes for instance doubles.

Library/Homebrew/test/cache_store_spec.rb

@@ -98,7 +98,7 @@ describe CacheStoreDatabase do
   describe "#close_if_open!" do
     context "database open" do
       before(:each) do
-        subject.instance_variable_set(:@db, instance_double("db", close: nil))
+        subject.instance_variable_set(:@db, instance_double(DBM, close: nil))
       end
 
       it "does not raise an error when `close` is called on the database" do

Library/Homebrew/test/cask/artifact/installer_spec.rb

 describe Hbc::Artifact::Installer, :cask do
   let(:staged_path) { mktmpdir }
-  let(:cask) { instance_double("Cask", staged_path: staged_path, config: nil) }
+  let(:cask) { instance_double(Hbc::Cask, staged_path: staged_path, config: nil) }
   subject(:installer) { described_class.new(cask, **args) }
   let(:command) { SystemCommand }