test: test_periodical_metadata_flush shrink timer period, kill app

Decrease the switch timer period to 100ms (instead of 1s), since we're
only getting 2s worth of app run. If the system is busy, nothing really
guarantee that the timer will indeed fire during this time-frame (so
strictly speaking, this test could still fail).

Also, kill the application before trying to read the trace: this should
ensure that the trace is not appended to concurrently with validation.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>