tests: drivers: uart: Explicitly test incremental receive length

Add an explict test for the length of the receive event when
doing incremental receives.  This was not tested anywhere in the
normal code path (only implicitly in the abort, which wasn't
exercised on the only current implementation, nrfx).  So add an
explicit check for it during the chained reads, so that the
test can catch the case where multiple receive events do not set
the length correctly on events past the first.

Tested on nrf52840_pca10056.

Signed-off-by: Derek Hageman <hageman@inthat.cloud>