pytest twister harness: increase timeout when stopping subprocess

When running pytest tests with coverage enabled, the test process starts
to write the coverage data when it receives the SIGTERM signal. On slow
machines, this can take longer than 0.5s. Increase the timeout and only
attempt to kill the process if it is still running after the timeout.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>