Remove the use of CMake's GoogleTest module

Using recent versions of CMake causes `gtest_discover_tests` to fail when parsing `jwt/tink_test_jwt_jwk_set_converter_test`, likely because of the issue in [1].
Using `add_test` avoids the issue at the price of losing some granularity (from per test case to per test binary).

[1] https://gitlab.kitware.com/cmake/cmake/-/issues/23039

PiperOrigin-RevId: 424321280
diff --git a/cmake/TinkBuildRules.cmake b/cmake/TinkBuildRules.cmake
index d71abc4..f2cdef1 100644
--- a/cmake/TinkBuildRules.cmake
+++ b/cmake/TinkBuildRules.cmake
@@ -34,11 +34,6 @@
 
 include(CMakeParseArguments)
 
-if (NOT ${CMAKE_VERSION} VERSION_LESS 3.9)
-  include(GoogleTest)
-endif()
-
-
 if (TINK_BUILD_TESTS)
   enable_testing()
 endif()
@@ -210,11 +205,9 @@
   set_property(TARGET ${_target_name} PROPERTY CXX_STANDARD ${TINK_CXX_STANDARD})
   set_property(TARGET ${_target_name} PROPERTY CXX_STANDARD_REQUIRED true)
 
-  if (${CMAKE_VERSION} VERSION_LESS 3.9)
-    add_test(NAME ${_target_name} COMMAND ${_target_name} WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
-  else()
-    gtest_discover_tests(${_target_name} WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
-  endif()
+  # Note: This was preferred over using gtest_discover_tests because of [1].
+  # [1] https://gitlab.kitware.com/cmake/cmake/-/issues/23039
+  add_test(NAME ${_target_name} COMMAND ${_target_name} WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
 endfunction(tink_cc_test)
 
 # Declare a C++ Proto library.