Cope with conditionally rendered form inputs

[s3cur3]

This fixes an issue with fill_in where past input values were accumulated and then passed to LiveViewTest.render_change/2 even when the input that would have produced the value was no longer present in the form. (The change simply removes inputs whose name is no longer present in the form when using fill_in.)

On a complicated form, you might have some inputs (say, a <select>) which conditionally hide other elements on the form. (One could argue that the unused inputs should merely be hidden, rather than removed from rendering in such cases, but on a sufficiently complicated form, it sometimes makes sense to truly remove the inputs.)

Prior to these changes, the newly added test would fail with a message like this:

  1) test conditional form fields submitting after switching versions only includes the visible field (PhoenixTest.LiveTest)
     test/phoenix_test/live_test.exs:1458
     ** (ArgumentError) could not find non-disabled input, select or textarea with name "version_a_text" within:

     <select id="version" name="version"><option value="a">Version A</option><option value="b">Version B</option></select><input id="version_b_text" name="version_b_text"/>
     code: |> fill_in("Version B Text", with: "some value for B")
     stacktrace:
       (phoenix_live_view 1.1.8) lib/phoenix_live_view/test/live_view_test.ex:1122: Phoenix.LiveViewTest.call/2
       (phoenix_test 0.9.1) lib/phoenix_test/live.ex:460: PhoenixTest.Live.trigger_form_phx_change/3
       test/phoenix_test/live_test.exs:1463: (test)

As a user, that error was quite confusing, because I said to myself "Well that doesn't make sense! I was trying to fill in 'Version B Text,' not version_a_text like the error message says!"

(With this change, that test simply passes.)