chore(form-select): ignore order of classes in test

What does this MR do?

As it was discovered during Vue3 migration &7203 - Vue 2 and Vue 3 renders classes in different order (Vue 2 is bringing classes which were put in class attribute when rendering component last, while Vue3 puts them first)

Since order of classes in class tag does not matter according to HTML spec, I'm intentionally relaxing related check in spec

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.