fix test

Library/Homebrew/test/formula_spec.rb

@@ -997,14 +997,15 @@ describe Formula do
     end
 
     it "returns false when set with a symbol" do
+      # Ensure that prefix does not match the default
+      stub_const "Homebrew::DEFAULT_PREFIX", "foo"
+
       f = formula "foo" do
         url "foo-1.0"
 
         pour_bottle? :default_prefix_required
       end
 
-      # Homebrew::DEFAULT_PREFIX is still /usr/local or /opt/homebrew
-      # and HOMEBREW_PREFIX is a temporary test directory
       expect(f).not_to pour_bottle
     end