devices: serial: add thr, dlab, modem and scratch unit tests
devices: serial: add thr, dlab, modem and scratch unit tests

Add thr, dlab, modem and scratch unit tests.
Extend input & output unit tests by also testing LSR reading
and reading/writing into/from a 2-length array. Also, when
testing the interrupt event fd, firstly, write 1 to it, so that
read doesn't block in case the event fd counter doesn't
change (for 0 it blocks).

Signed-off-by: Constantin Musca <e6b0bba089ab6d3e8fde8b62faae63e6b21d7743@amazon.com>

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