Tests: Add more PHPUnit tests for form type classes

[priethor]

What

Follow-up to #333. Adds additional PHPUnit tests for form type classes.

How

File	Tests Added
test-form-comment	test_edit_comment_renders_nothing_without_field_groups
test-form-front	test_enqueue_form_calls_check_submit_form, test_submit_form_applies_pre_submit_filter, test_submit_form_fires_action, test_submit_form_sets_global
test-form-gutenberg	test_add_meta_boxes_removes_edit_form_after_title_action, test_block_editor_meta_box_hidden_fields_method_exists
test-form-nav-menu	test_admin_enqueue_scripts_bails_when_not_on_nav_menus_screen
test-form-taxonomy	test_admin_enqueue_scripts_bails_on_wrong_page
test-form-user	test_admin_enqueue_scripts_bails_when_not_on_user_screen, test_render_edit_renders_nothing_without_field_groups, test_render_edit_with_different_user_ids
test-form-widget	test_admin_enqueue_scripts_bails_when_not_on_widgets_screen

Testing Instructions

Run the form type tests:

vendor/bin/phpunit --filter="Test_Form"

[github-actions]

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Props priethor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.89%. Comparing base (af9ae43) to head (af4b1f4).
⚠️ Report is 4 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #347      +/-   ##
============================================
+ Coverage     31.71%   31.89%   +0.17%     
  Complexity     4410     4410              
============================================
  Files           291      291              
  Lines         36918    36914       -4     
============================================
+ Hits          11710    11775      +65     
+ Misses        25208    25139      -69     

Flag	Coverage Δ
e2e	42.91% <ø> (-0.51%)	⬇️
javascript	15.07% <ø> (ø)
phpunit	29.08% <ø> (+0.37%)	⬆️

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