Replaced usage of toBeTruthy and toBeFalsy

What does this MR do and why?

• Replaced usage of toBeTruthy and toBeFalsy in spec/frontend/design_management/pages/index_spec.js

• Resolves the issue

Screenshots or screen recordings

How to set up and validate locally

1. Inside gitlab directory, Run

   yarn jest spec/frontend/design_management/pages/index_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #369774 (closed)