Rename ABSL_*_IMPL macros to match the other ABSL_LOG_INTERNAL_* macros and to make sure it's clear that they're internal.

Also rename the log and check test files from .h to .inc per https://google.github.io/styleguide/cppguide.html#Self_contained_Headers.

PiperOrigin-RevId: 533603350
Change-Id: Iad5d8b683e33b63784cc8e64b84da09f5fc3bf1e