Add unit tests for block registration with RTL stylesheets

[SergeyBiryukov]

Background: #56325

[54330] should have some unit tests for register_block_style_handle() to confirm that RTL stylesheets for blocks are correctly loaded when an RTL locale is set.

[thomasplevy]

@SergeyBiryukov how's the above patch to cover the RTL-related code?

This adds a PHPUnit test for register_block_style_handle() to confirm that RTL stylesheets for blocks are correctly loaded when an RTL locale is set.

Refreshes the previous patch.

Trac ticket: https://core.trac.wordpress.org/ticket/56797

[costdev]

@SergeyBiryukov I've submitted ​PR 4150 which refreshes 56797.patch to improve the docblock, to reset the text direction before the assertions, and to add $message parameters to the assertions.

I ran a coverage report, and can confirm that the test covers ​the related lines - ​Screenshot.

[costdev]

This ticket was discussed during the bug scrub. @SergeyBiryukov, do you have time to take a look at ​PR 4150 before RC1 on Thursday 9th March?

Additional props: @mukesh27

[SergeyBiryukov]

In 55486:

Tests: Add a unit test for register_block_style_handle() with an RTL locale.

The test ensures that the function loads RTL stylesheets when a locale with the right-to-left text direction is set.

Follow-up to [54330].

Props costdev, thomasplevy, audrasjb, mukesh27.
Fixes #56797.

Thanks for the PR! Merged in r55486.