Set GTEST_BOTH_LIBRARIES appropriately (#1669)

If `find_package()` succeeds to find GTest and `INSTALL_DEPENDENCIES`
is set to OFF, `GTEST_BOTH_LIBRARIES` is not set and thus
`rccl-UnitTests` fails with trying to link unkown symbols.

cmake/Dependencies.cmake

@@ -70,6 +70,8 @@ if(NOT GTest_FOUND AND BUILD_TESTS OR INSTALL_DEPENDENCIES)
         message(FATAL_ERROR "Cannot find gtest library installation path.")
     find_package(GTest REQUIRED CONFIG PATHS ${GTEST_ROOT})
     endif()
+elseif(GTest_FOUND AND BUILD_TESTS)
+  set(GTEST_BOTH_LIBRARIES "GTest::gtest;GTest::gtest_main")
 endif()
 
 # Find or download/install rocm-cmake project