tests: revert modification to ARGV at end of test

Similar to 6fc1747e0550545b4fe80f5a1854d16dd2b08afe. I hadn't seen any
test failures caused by this, but I noticed it in the process of
trackign that down and it's an accident waiting to happen.

Library/Homebrew/test/formula_installer_test.rb

@@ -61,6 +61,8 @@ class InstallTests < Homebrew::TestCase
       assert_equal 3, bin.children.length
       assert_predicate f.prefix/".brew/testball.rb", :readable?
     end
+  ensure
+    ARGV.reject! { |a| a == "--with-invalid_flag" }
   end
 
   def test_bottle_unneeded_formula_install