New port submission: libgtest-dev

[carlesfernandez (Carles Fernandez)]

Google's framework for writing C++ tests on a variety of platforms, based on the xUnit architecture. Supports automatic test discovery, a rich set of assertions, user-defined assertions, death tests, fatal and non-fatal failures, value- and type-parameterized tests, various options for running the tests, and XML test report generation.

This port downloads the source code and copies it at ${prefix}/src, so other ports can use it. This is required for instance by gnss-sdr (see ​https://trac.macports.org/ticket/46551). No building or installing steps are done.