tests: drivers: uart: Fix async write abort test

During the write abort test, a second write is started then quickly
aborted.  This means that the number of bytes sent is relative
to that second write.  So when comparing it against the number
of bytes received which is NOT reset, the first (completed)
send of five bytes has to be accounted for.  The current
nrfx implementations abort quick enough that no bytes are reported
sent, so this wasn't currently being exercised (i.e. the short
circuit of zero bytes sent was taken).

Tested on nrf52840_pca10056.

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