Webfonts: change class properties from static to instance members

[zaguiini]

Part of #39332.

What?

Having static variables in the WP_Webfonts class makes the class share state between instances. This is not needed in a lot of cases and @jeyip and I couldn't figure out why it was done like that.

Why?

That are several problems with that approach, including shared state issues. The tests become aware of each other and order starts to matter. Other instances of WP_Webfonts might appear in the codebase, and that might lead to confusing and unexpected behavior. It's even impossible to test features of the wp_webfonts_* API in the current state of things.

I wonder why we decided to take this route 🤔

How?

By moving $webfonts and $providers variables to instance members, whenever someone creates a new instance of WP_Webfonts, they'll start with a fresh copy.

We're also moving from static to global inside wp_webfonts so the instance can be accessed and reset before running a new test case.

Testing Instructions

Nothing should change at the implementation level. You should notice the test's expectations are a little different, but the tests are still passing.

[zaguiini]

@aristath please review that as soon as possible as it's blocking us from writing new tests to #39327. Thanks!

[aristath]

I'm on the fence about this one.
I understand why we want to do it... But introducing a new singleton class just for the sake of tests - with no other value to the implementation itself - seems a bit weird.
The wp_webfonts() function with the use of a static $instance var does pretty much the same thing a singleton does, so in real-life there would never be more than 1 instances of the class.

What if instead of introducing a new _WP_Webfonts_Singleton object, we just added a new reset_instance() method in the WP_Webfonts class?
Or we could make the $webfonts and $providers public instead of private

Both solutions above would allow us to reset the class props during tests, without the addition of a new singleton class 🤔

[zaguiini]

@aristath,

I moved from static to global and now we can intercept the $wp_webfonts global variable before each test runs. That's how wp_styles() is doing and it's the norm in the codebase. Hope it looks good to you.

[aristath]

LGTM! 👍

[zaguiini]

Test failures are not related to the changes proposed by the PR. End to End Tests are failing regularly on trunk.