Permit installation of googletest

[rleigh-codelibre]

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Closes: #8935

Please see #8935 for a full description of the change. This PR removes the exclusions for gtest and gmock so that it will be possible to install google test in the future. The prohibition upon installing these systemwide is no longer in place, as detailed in the issue description.

[MikeMcQuaid]

Thanks for the PR @rleigh-codelibre!