Tink CMake: Simplify the handling of tests.

The previous syntax for GTEST_ADD_TESTS only works with later versions of cmake, see e.g. https://cmake.org/cmake/help/v3.7/module/FindGTest.html?highlight=gtest_add_tests

Since we use cmake 3.7.1 in kokoro we cannot run these tests like this. Furthermore, the function gtest_add_tests is problematic for parametrized tests. Hence, we remove it and replace it with the simpler add_tests.

PiperOrigin-RevId: 253555478