Factor out MSVC-only `version.rc` logic, and ensure other variables are always initialized (#9880)

diff --git a/cmake/tests.cmake b/cmake/tests.cmake
index 789ae4f..1f8bbc9 100644
--- a/cmake/tests.cmake
+++ b/cmake/tests.cmake
@@ -245,6 +245,8 @@
     string(APPEND protobuf_TEST_XML_OUTDIR "/")
   endif()
   set(protobuf_GTEST_ARGS "--gtest_output=xml:${protobuf_TEST_XML_OUTDIR}")
+else()
+  set(protobuf_GTEST_ARGS)
 endif()
 
 add_executable(tests ${tests_files})