tests: logging/dictionary: pytest only looks at last output

There are instances where on hardware tests would result in
multiple dictionary logging blocks. This is usually due to
flashing and running via twister in separate steps. Once
flashing is done, the device starts running and sending
logging string to output, but twister is not ready to read
them, thus leaving the strings in the host's UART buffer.
Then twister starts its testing run by resetting the device
which results in another dictionary logging block to show
up. So the pytest script has to be updated to only look at
the last dictionay logging block.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>