Clear fields in tests before filling them in

There seems to be an issue with capybara or chromedriver which results
in `fill_in` sometimes appending to an input rather than overwriting
[1], causing some tests to fail under certain circumstances.

Clearing fields before filling them in solves the issue.

Note we're now getting warnings on all tests using the rack driver. I
haven't found a way to avoid the `clear: :backspace` option in
non-JavaScript tests, so to avoid the annoying warnings we should reduce
the number of tests using the rack driver even more.

[1] See issue 2419 in https://github.com/teamcapybara/capybara/issues

spec/rails_helper.rb

@@ -47,6 +47,7 @@ end
 
 Capybara.exact = true
 Capybara.enable_aria_label = true
+Capybara.default_set_options = { clear: :backspace }
 Capybara.disable_animation = true
 
 OmniAuth.config.test_mode = true