Add missing InitGoogleTest line in "Registering tests" example code
Copying the original code gives the following error message
"""
IMPORTANT NOTICE - DO NOT IGNORE:
This test program did NOT call testing::InitGoogleTest() before calling RUN_ALL_TESTS(). This is INVALID. Soon Google Test will start to enforce the valid usage. Please fix it ASAP, or IT WILL START TO FAIL.
"""
PiperOrigin-RevId: 408385714