tests: robot: renode: Use precise pause for UART keywords

Remove the source of non-determinism during emulation
by keeping terminal events reproducible between runs.
By using the mechanism of precise pause in Renode emulator
every interaction with terminal happens at the same virtual time
from the perspective of an emulated system, hence makes it independent
of the host. Otherwise the result of a test is susceptible to
the load on the host and robot process keywords' timing.

Renode emulator supports precise pausing when specified sequence of
characters is received on a terminal. Besides that,
char delay parameter is specified to set time interval between
consecutive characters entered in the terminal.
'Start Emulation' keyword is redundant, because 'Wait For'
keywords automatically continue emulation up to the nearest pause
event (e.g. matched pattern on the terminal).

Signed-off-by: Marek Slowinski <mslowinski@antmicro.com>