For some reason lnd doesn't properly shut down on some windows itests,
and the process still running makes it impossible to access its log
files.
So for the Windows integration test, we attempt to kill the whole
process tree once the process indicates it is done.
When one of the itest tranches fails, because we are running tests in
parallel, other tranches will still be running, which caused 7z to fail
at zipping the logs because race reads. This commit fixes it by making
sure we are waiting other tranches to finish before moving to zipping
and uploading the logs.
