CMake: unit-tests: Fetch googletest master

googletest now follows the "Abseil Live at HEAD" philosophy, which means
they recommend using the latest commit on the master branch and always
compiling from source. They recommend this to avoid version mismatch
issues and "diamond dependency" problems which are common in dependency
graphs with pinned versions. Google make the "promise" that future
changes won't break downstream code if it follows the "Abseil
compatability guidelines".

Upping the version to master also fixes some CMake configure time
warnings that were present with the older tagged releases of googletest:

CMake Deprecation Warning at __build/_deps/googletest-src/CMakeLists.txt:4 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

CMake Deprecation Warning at __build/_deps/googletest-src/googlemock/CMakeLists.txt:45 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

CMake Deprecation Warning at __build/_deps/googletest-src/googletest/CMakeLists.txt:56 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

UNITTESTS/CMakeLists.txt

@@ -19,7 +19,7 @@ include(FetchContent)
 # Download and unpack googletest
 FetchContent_Declare(googletest
     GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG release-1.10.0
+    GIT_TAG master
 )
 FetchContent_MakeAvailable(googletest)