twister: pytest: Fix problems with no prompt in tests with shell

Only impacts on twister-pytest scenarios. Exec command is waiting for
the prompt message, but in some cases, e.g. after reseting the DUT, prompt
can be followed by another strings. Solution is to clear internal serial
buffer and to not request the prompt string before executing a command.

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>