I rolled my own.. I think I may have even redrawn a schematic and posted it here at least once...
Basically I made a simple on/off fet shunt (like a noise gate). I carried it one step further and detected zero crossings, so I could always start and stop on a zero crossing.
I then added a pot to feed signal around the gated off part for the baseline or off amplitude.
Then just add a signal from a sine wave generator or whatever signal source you want to use.
JR
PS: The zero crossing switching detection is useful when designing dynamics circuitry by ear, otherwise the clicks from the tone burst would mask any clicky artifacts from the processing. I also grabbed the gate drive signal to trigger my scope.
PPS: A rule of thumb for measuring rise time time constants is 90% of final value... this is also close to within 1dB, so I suspect either criteria is adequate for quantifying. Some comp circuits have signal dependent or adaptive att/rel so may not give uniform time constants in all cases. I also used a hold time before release in some designs so that needs to be looked at separately from the release time.