ENH: Update GTest branch name to `master` in order to "Live at Head"
ENH: Update GTest branch name to `master` in order to "Live at Head"

https://github.com/google/googletest#live-at-head

Policy change occurred in March:
https://github.com/google/googletest/commit/861d535e2595894c7a214f05c1578d964a617643

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