Enable prefer form input filter flag by default

- This fixes a regression in 2.2.3 via #4996, whereby the fix to a
  regression caused a new regression in behavior, in which the form
  input filters were no longer being merged in the expected order.
  Essentially, the fix in #4996 now enforced the `preferFormInputFilter`
  flag status, which was de facto enabled due to the hacks previously
  present. This flag is now enabled by default to provide the behavior
  by default (which is what was occurring before the changes, albeit via
  a different mechanism).
Add unit test group to new test
Merge branch 'hotfix/prefer-form-input-filter-flag-default'

Close #5028