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.