Add GTest dependency if not found

[YanzhaoW]

Changes

1. Add Googletest dependency using cmake Fetchcontent if it's not found in the local system.
2. Change the gtest lib name from ${GTEST_BOTH_LIBRARIES} to GTest::gtest_main. The former one has been deprecated in newer versions.

Fixes

1. Close the issue #738
2. Fix the PR #840 regarding being unable to run gtest.

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Checklist:

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

[YanzhaoW]

@jose-luis-rs If there is no other problems for this PR, please merge it.