[Fonts API] Add tests for gutenberg_add_registered_fonts_to_theme_json()

[hellofromtonya]

What?

Adds PHPUnit tests for gutenberg_add_registered_fonts_to_theme_json().

And as this new test also includes the same switch_theme() fixtures code, moved it to the TestCase for reuse and updated the other test accordingly.

Why?

• To provide code coverage specifically for this function as it is critical for the automatic registration and enqueuing of the theme's theme.json and style variation fonts and variations.
• To set a baseline of automated tests before improvement happens for this function.

How?

Adds a test class for this function with the following coverage:

• Ensure an instance of WP_Theme_JSON_Gutenberg is returned.
• Returns the given instance (bails out) when there are no fonts to add.
• Adds registered fonts not included in the theme's theme.json.

Notes:

In writing these tests, discovered the following bugs:

• [Fonts API] When not in Site Editor, only enqueued fonts should be added to global styles in gutenberg_add_registered_fonts_to_theme_json() #50047
• [Fonts API] Registered variation for font in theme not added in gutenberg_add_registered_fonts_to_theme_json()  #50048

The data set for #50048 is included but commented out.

Testing Instructions

CI is green.

Manual

npm run test:unit:php -- --filter Tests_Fonts_GutenbergAddRegisteredFontsToThemeJson

or all of the Fonts API tests by:

npm run test:unit:php -- --group fontsapi

[anton-vlasenko]

The code looks good to me.
I only have one question regarding the unused code in the data provider (please see below).
Thank you.

[anton-vlasenko]

The code looks good to me.
I've tested this PR using the command

php vendor/bin/phpunit -c phpunit.xml.dist --filter Tests_Fonts_GutenbergAddRegisteredFontsToThemeJson

and the tests passed.
So, I approve this pull request.

[hellofromtonya]

Thank you @anton-vlasenko!