[FIX] Fix CMake configuration when GoogleTest doesn't exist

[YanzhaoW]

Remove REQUIRED from find_package of GTest.

Fix #1080

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Checklist:

• [x] Rebased against dev branch
• [x] My name is in the resp. CONTRIBUTORS/AUTHORS file
• [x] Followed the pull request guidlines and the Git workflow
• [x] Followed the seven rules of great commit messages