emoji tests: make sure to unset install badge env vars

Otherwise the user's specified emoji will be used and the test will
fail.

Library/Homebrew/test/emoji_spec.rb

@@ -4,6 +4,11 @@ describe Emoji do
   describe "#install_badge" do
     subject { described_class.install_badge }
 
+    before(:each) do
+      ENV.delete("HOMEBREW_NO_EMOJI")
+      ENV.delete("HOMEBREW_INSTALL_BADGE")
+    end
+
     it "returns 🍺 by default" do
       expect(subject).to eq "🍺"
     end