How are high-speed circuits tested if test equipment doesnt exist?
For some perspective, consider that optical signals are still too high frequency for the instantaneous electric field to be sampled and measured, but there are still lots of different kinds of measurements we can do on an optical signal.


With a power sensor (a photodiode or even an LDR) we can measure the power of the signal.
With a prism or diffraction grating we can build a spectrometer and get a rough idea of the signal's spectrum and/or pulsewidth.
With an interferometer we can mix the optical signal with a delayed version of itself and measure the coherence time (bandwidth) of the signal with perhaps gigahertz resolution.
With a tunable local oscillator (laser), we can even down-mix the signal and measure its spectrum with an RF spectrum analyzer, getting 100's of kHz resolution.


All of these measurements have analogs in the microwave regime and were or could be used by microwave engineers prior to the advent of multi-gigahertz oscilloscopes.