Fix test failure

Author: carlocab

Library/Homebrew/test/utils/git_spec.rb

@@ -186,7 +186,10 @@
       unless ENV["HOMEBREW_TEST_GENERIC_OS"]
         it "installs git" do
           expect(described_class).to receive(:available?).and_return(false)
-          allow_any_instance_of(Formula).to receive(:ensure_installed!)
+          allow(CoreTap.instance).to receive(:installed?).and_return(true)
+          formula_double = instance_double(Formula)
+          allow(Formula).to receive(:[]).with("git").and_return(formula_double)
+          allow(formula_double).to receive(:ensure_installed!).and_return(formula_double)
           expect(described_class).to receive(:available?).and_return(true)
 
           described_class.ensure_installed!