[battery_plus_macos] Add integration test

[Nash0x7E2]

Battery Plus macOS is currently missing integration tests.

Objectives

• Add test cases for getBatteryLevel
• Add test cases for onBatteryStateChanged

[miquelbeltran]

The e2e tests seem to be located in https://github.com/fluttercommunity/plus_plugins/tree/main/packages/battery_plus/test, which already checks for batterylevel. Have you tried to run that test on MacOS?

(also, the test location is wrong, as I reported in #28)