Allow registering custom categories through filter

[ernilambar]

Fixes #494

• Introduce filter wp_plugin_check_categories
• Add new method get_check_categories() in Check_Categories class
• Add test for related changes

Example implementation: https://github.com/ernilambar/fresh-check/blob/main/fresh-check.php

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Co-authored-by: ernilambar <rabmalin@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
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[ernilambar]

Looks like we have used key/value pair, like:

$category_labels  = $check_categories->get_category_labels();

$categories = array();

foreach ( $category_labels as $slug => $label ) {
	$categories[] = array(
		'slug' => $slug,
		'name' => $label,
	);
}

So I think we need to adjust in other places also.

[swissspidy]

Then these places should call get_categories()

[mukeshpanchal27]

In 37aeb20, I have removed the test_get_category_labels test as they test the same get_categories() data values, making them redundant.

[ernilambar]

May be we could remove that labels function. It is not used anywhere I guess.

[mukeshpanchal27]

Yes but that function introduce in 1.0, leave it for now.