Adds tests in base_token_spec - showSuggestions

What does this MR do and why?

This MR introduces new tests that should solve for #336791 (closed)

The computed property showSuggestions is responsible for showing/hiding the suggestions template. These tests aim to lock down the behavior of showSuggestions via testing the methods that compose it.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. yarn jest --watch spec/frontend/vue_shared/components/filtered_search_bar/tokens/base_token_spec.js

MR acceptance checklist

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