[rubygems/rubygems] Improve uninstaller tests

Tests have two copies of b-2 installed, one in default home and another
in user home. But only the one in default home was visible because of
not reloading paths. The user install test was working by chance.

rubygems/rubygems@667537a08f

test/rubygems/test_gem_uninstaller.rb

@@ -429,7 +429,7 @@ def test_uninstall_not_ok
   end
 
   def test_uninstall_user_install
-    @user_spec = Gem::Specification.find_by_name "b"
+    Gem::Specification.dirs = [Gem.user_dir]
 
     uninstaller = Gem::Uninstaller.new(@user_spec.name,
                                        executables: true,