Renaming/reordering/IWYU in repeated_ptr_field_unittest.cc

* Renamed the test suite from `RepeatedPtrField` to `RepeatedPtrFieldTest` (that enables making it a friend of `RepeatedPtrField` proper if necessary, e.g. for debugging, among other things).
* Added a missing infix in some microbenchmark names enables easy filtering to run only `RepeatedPtrField`-related ones.
* Added a couple basic tests for `Add()`.
* Aggregated all generic, non-RepeatedPtrField BMs in the bottom of the source.

PiperOrigin-RevId: 741231691

Author: protobuf-github-bot