Fix $grid-breakpoints SCSS unit tests init

[julien-deramond]

Description

This PR changes the values of $grid-breakpoints in scss/tests/utilities/_api.test.scss to be sure that this value is well overriden. Since it was not the case, also added a !global.

If you need to manually test this modification, you can remove the !global and you'll see that even if we expect to change the small and medium values respectively to 333 and 666, the test still passes.

Motivation & Context

Unit test worked because $grid-breakpoints contained the same values for small and medium breakpoints so we didn't detect that the overriden value didn't work.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• (N/A) My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed