sanitycheck: run custom pre script before open the serial port

In some cases it might be a good idea to reset the board for real to
make sure it is completely recovered from some failed state (simple
re-loading of the application binary or even Elf file contents doesn't
affect most of internal CPU states so doesn't help in recovery,
see https://github.com/zephyrproject-rtos/zephyr/issues/25022 &
https://github.com/zephyrproject-rtos/zephyr/issues/26665).
And so we may want to utilize some external utility which triggers the
hard reset (in case of ARC boards it is
https://github.com/foss-for-synopsys-dwc-arc-processors/rff-ftdi-reset

).
So we need to have a way to execute an external command before each and
every test.

Now given we already have quite some call-backs we try to use them
before re-inventing the wheel. And pre_script seem to be a good option
with just on minor note - it is called after serial port gets open.
And while in some cases it might be OK if serial port on the board is
not affected by the board's reset, if it is affected we'll be losing
connection on reset (and that's the case with ARC boards BTW as the
FTDI USB-to-Serial IC is also wired to the reset signal on most of
the boards). That said we just move invocation of pre_script before
opening the serial port and everything should be good now.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>