Introduce googletest library

The library will be either found in the system or downloaded via FetchContent.

CMakeLists.txt

@@ -2507,6 +2507,21 @@ if(ENABLE_TESTS)
   # meta-target: each test is supposed to add_dependencies(tests ${NEWTEST})
   # then, it is possible to build and execute all tests using "ninja tests && ctest"
   add_custom_target(tests)
+  find_package(GTest)
+  if (NOT GTest_FOUND)
+    message(STATUS "GTest package not found, will download googletest automatically. To prevent that install google test on your system (libgtest-dev)")
+    include(FetchContent)
+    FetchContent_Declare(
+      googletest
+      GIT_REPOSITORY https://github.com/google/googletest.git
+      GIT_TAG        58d77fa8070e8cec2dc1ed015d66b454c8d78850 # 1.12.1
+    )
+    set(BUILD_GMOCK OFF)
+    set(INSTALL_GTEST OFF)
+    FetchContent_MakeAvailable(googletest)
+    add_library(GTest::gtest ALIAS gtest)
+    add_library(GTest::gtest_main ALIAS gtest_main)
+  endif()
 endif()
 
 add_subdirectory(common)