How to quantify testability
When I worked in this field,  years ago, building functional test systems, one of the things I'd do was sit down with a printout of the schematic, and some colored pens. For each test, I'd color in the part of the schematic that it verified.
Power up test - color in the power supply section.
Device programming - color in the flash memory and related programming connections.
Boot up - color in the MCU, Flash, and memory bus.
etc....
I'm sure the tools available have improved in the intervening years.

Keep in mind that different test methods have tradeoffs, usually related to cost or speed.
Flying probe is slow, but has low tooling cost.
ICT has high tooling cost, but is fast.
ICT or flying probe may not be able to physically reach every circuit node.
Functional test may allow you to verify circuits that you can't probe with ICT or Flying Probe.