Add test(s) for dynamic value prop in GlFilteredSearch

The following discussion from !2504 (merged) should be addressed:

• @fguibert started a discussion: (+10 comments)

  @kushalpandya Could you please review this small change? I'm also not sure how this can be tested in gitlab-ui on its own so let me know if you have a suggestion.

  Adding a test which does something like await wrapper.setProps({ value: someNewValue }) after the wrapper has already been created an initialised with some other value, and asserting the expected new token(s) get rendered would be a good approach 👍