Mocking Hardware
Unit Testing Firmware drew a clean line around the logic that needs nothing from the hardware at all — a parser, a state machine, a checksum — and pointed at this page for the harder case: logic whose whole job is to talk to a peripheral. A driver cannot be tested by giving it inputs and checking outputs the way a parser can, because its inputs and outputs are register writes and register reads, and there is no register to read from on a laptop. Testing it on the host requires something on the other end of the bus that behaves like the real device without being it.
Unit Testing Firmware
The ordinary embedded development loop is: change a line, build, flash, power-cycle or reset, and watch a UART or an LED to see whether the change did what you meant. On a fast board with a fast probe that loop is a few seconds; on a slow one, or one shared with a bench full of other work, it is tens of seconds, and every one of those seconds is spent re-verifying code paths that had nothing to do with the line you just changed. A state machine with a dozen transitions and a handful of edge cases does not get thoroughly exercised at that cadence — it gets exercised until the one case someone thought to try passes, and the rest ride along untested until a customer finds them.