Balance mega tests, shorten some env names

.github/workflows/test-builds.yml

Marlin/src/pins/pins.h

Marlin/src/pins/rambo/pins_MINIRAMBO.h

buildroot/share/git/mftest

buildroot/share/tests/STM32F103RC_btt-tests

buildroot/share/tests/STM32F103RC_btt_USB-tests

buildroot/share/tests/STM32F103RE_btt-tests

buildroot/share/tests/STM32F103RE_btt_USB-tests

buildroot/share/tests/mega1280-tests

buildroot/share/tests/mega2560-tests

buildroot/share/tests/sanguino1284p-tests

buildroot/share/tests/sanguino644p-tests

buildroot/share/vscode/auto_build.py

platformio.ini