emul: Support getting an emulator by name

Issue #38271

Implement a getter for emulators similar to device_get_binding. This
function can be used to get the emulator instance during tests to call
emulator specific functions.

Example: The current BMI160 emulator pre-defines a finite set of data
samples that will be returned. If a test was to be written for logic
that uses that data, then the emulator would become completely useless
without the ability for the test to define what data should be returned.

This will also help in exercising error conditions in tests.

Signed-off-by: Yuval Peress <peress@chromium.org>