Test if the user dir is used for auto user installation

This is mainly to align this test case with the
`test_process_options_does_not_fallback_to_user_install_when_gem_home_
not_writable_and_no_user_install`, where the `install_dir` is checked
already.

test/rubygems/test_gem_installer.rb

@@ -1987,8 +1987,9 @@ def test_process_options_fallback_to_user_install_when_gem_home_not_writable
 
     FileUtils.chmod 0o000, @gemhome
 
-    use_ui(@ui) { Gem::Installer.at @gem }
+    installer = use_ui(@ui) { Gem::Installer.at @gem }
 
+    assert_equal Gem.user_dir, installer.gem_home
     assert_equal "Defaulting to user installation because default installation directory (#{@gemhome}) is not writable.", @ui.output.strip
   ensure
     FileUtils.chmod 0o755, @gemhome