Use system GTest instead of fetching it
https://bugs.gentoo.org/880353
--- a/test/CMakeLists.txt	2025-03-23 22:20:35.060895402 +0300
+++ b/test/CMakeLists.txt	2025-03-23 22:22:29.964436254 +0300
@@ -33,12 +33,7 @@
 set(BUILD_GMOCK OFF CACHE INTERNAL "Override gtest default" FORCE)
 set(INSTALL_GTEST OFF CACHE INTERNAL "Override gtest default" FORCE)
 
-include(FetchContent)
-FetchContent_Declare(gtest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG v1.15.2
-)
-FetchContent_MakeAvailable(gtest)
+find_package(GTest REQUIRED)
 
 add_executable(st_gtests "")
 target_link_libraries(st_gtests PRIVATE gtest gtest_main string_theory)