emul: Introduce emulator backend API and generic sensor test

This PR introduces a backend API to be implemented by sensor emulators
that creates a standardized mechanism for setting expected sensor
readings in tests. This unlocks the ability to create a generic sensor
test that can automatically set expected values in supported sensor
emulators and verify them through the existing sensor API. An
implementation of this API is provided for the AKM09918C magnetometer.

A generic sensor test is also created to exercise this implementation.
Observe that this test knows nothing about the AKM09918C; info about
supported channels and sample ranges is discovered through the backend
API. The test iterates over all devices attached to the virtual I2C and
SPI buses in the test binary's device tree, which (theoretically) covers
all sensors. Sensors whose emulator does not exist yet or does not
support the backend API are skipped.

Signed-off-by: Tristan Honscheid <honscheid@google.com>