grid: fix "log buildout output in realtime"
this fixes 86ce8b8e which had an error the current_output variable was not reset when the current output ended in \n.
As a result, logged output was sometimes containing some parts of the program output twice. We could even catch it in unit test, when running the test multiple times. It also happened on test node with https://erp5js.nexedi.net/#/test_result_module/20210826-1706D7F50/10
The is trivial, we reset the current output also in this case. The test suite is extended a bit, with added tests and test adjustments made during the debugging. With test_exec_large_output_multiline the problem triggers almost every time.