this reports errors at the same location the blocking uart would, which
works out to being mostly exact (except in the case of overruns, where
one extra character is dropped). this is actually easier than going
nuclear in the case of errors and nuking both the buffer contents and
the rx fifo, both of which are things we'd have to do in addition to
what's added here, and neither are needed for correctness.
the rp uart receive fifo is 32 entries deep, so the 31 byte test data
fits into it without needing any buffering. extend to 48 bytes to fill
the entire fifo and the 16 byte test buffer.
