HP 3310B Malfunction Generator

They exist.

The device/instrument is commonly known as a "stimulator".

You would find them in places like huge anechoic chambers, providing (stimulating)simulated signals within the chamber. They typically also involve methods to properly monitor and observe said stimulations.

Whether it is a hard wired signal or an RF radiation, the process is still known as stimulation, when a non-real signal is 'created' to facilitate functional testing or 'malfunctory' diagnostic analysis.

Nearly all are custom to the scenario and gear.

Even more so than say your average HV supply, which is almost always custom to the application as well. There very few 'off-the-shelf' HV supplies and none that are broadly load dynamic such that there could be.

Stimulation is far worse. Zero off the shelf 'jobs'.

All are custom to the application and types of signals both being stimulated as well as those circuit elements being monitored for response to the stimulus. Timings are critical typically, so line lengths, etc. all have to be monitored and adjusted for with phase delay and frequency shift calibration elements, all so that the monitoring takes place such that a snapshot of the stimulus and a snapshot of the monitored response are timed together to make the gathered data usable as information.