tests: allow using system gtest copy

Distributions often do builds with no network access available
for both security reasons and also to ensure reproducibility.

This change tells CMake to query the system for a copy of gtest,
but if it's not available, it'll fall back to downloading via
FetchContent.

tests/CMakeLists.txt

@@ -3,6 +3,7 @@ include(FetchContent)
 FetchContent_Declare(
     googletest
     URL https://github.com/google/googletest/archive/9a32aee22d771387c494be2d8519fbdf46a713b2.zip
+    FIND_PACKAGE_ARGS NAMES GTest
 )
 
 # For Windows: Prevent overriding the parent project's compiler/linker settings
@@ -44,7 +45,8 @@ target_link_libraries(
     ELFIOTest
     PRIVATE
     elfio::elfio
-    gtest_main)
+    gtest_main
+    GTest::gtest_main)
 
 add_test(
     NAME