In the testing repository is the latest version of the Arduino core - 1.8.9. Along with that is an update to the AVR compiler AVR-GCC bringing it to 5.4.0 inline with the Arduino IDE's latest release.
But that's not all: we have just added AVR-GCC 9.1.0 to the testing repository! Yes, you heard it - AVR-GCC 9.1.0.
If you fancy trying out either of these new additions just add the testing repository in the plugin manager. AVR-GCC will then be upgradable to 5.4.0, and the Arduino core to 1.8.9 - both automatically. AVR-GCC 9.1.0 is an additional compiler you can install from the list.