GoogleTest is fetched by tag, not by commit

[skipbit]

test/CMakeLists.txt fetches the test framework by a movable ref:

FetchContent_Declare(
    googletest
    GIT_REPOSITORY https://github.com/google/googletest.git
    GIT_TAG        v1.15.2
)

A tag can be moved or deleted upstream, so two builds of the same commit of this repository can pull different sources. Pinning GIT_TAG to the commit hash makes the dependency reproducible; a hash comment naming the tag keeps it readable.