Replaced toBeTruthy and toBeFalsy with specific matchers

What does this MR do and why?

This merge request addresses issue #369659 (closed)

1. Line 79 changed matcher toBeTruthy to toBeInstanceOf(GitLabDropdown) which is the instance type of test.sidebarMoveIssue.$dropdownToggle.data('deprecatedJQueryDropdown').

2. Changed matcher toBeTruthy or toBeFalsy to toBe(true) or toBe(false) respectively for testing props

Screenshots or screen recordings

n/a

How to set up and validate locally

run yarn jest

MR acceptance checklist

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