Use page.find instead of within in component tests

In component tests, the `within` method is actually an alias to RSpec's
`be_within` matcher, which is used to test numeric ranges. That meant
the tests always passed, even when there were bugs on the page.

In order to use `within` in component tests, we have to use
`page.within`. However, that also fails, since there's no such method
for `Capybara::Node::Simple'` objects, which are used in component
tests.

So we're using `page.find` instead.

See also pull request 945 in https://github.com/github/view_component

spec/components/admin/budgets_wizard/headings/group_switcher_component_spec.rb

@@ -33,9 +33,9 @@ describe Admin::BudgetsWizard::Headings::GroupSwitcherComponent do
     expect(page).to have_content "Showing headings from the Parks group"
     expect(page).to have_button "Manage headings from a different group"
 
-    within "button + ul" do
-      expect(page).to have_link "Recreation"
-      expect(page).to have_link "Entertainment"
+    page.find("button + ul") do |list|
+      expect(list).to have_link "Recreation"
+      expect(list).to have_link "Entertainment"
     end
   end
 end