The test button does exactly the same thing. Neither the test button nor your meter is a reliable test of an RCD.
As stated previously, it's just a convenient go/no go test to see if the unit actually trips from various locations,
rather than having faith that just hitting on the TEST button is enough, and assume the premises have perfect wiring and no dramas at all the outlets
FWIW I've replaced 4 pole RCDs, used in 3 phase and single phase apps, that tripped fine from the outlets, yet the onboard TEST button failed to trip the unit,
and vice versa with other units. And no, it wasn't a Neutral mis-wire, which I always check.
Merely replacing those units with the same type/model, and wired in exactly as they were, fixed all issues
and passed any/all testing procedures with correct specified mA/Time/Polarity specs
i.e. a Low-Z meter, AVO 8 or 7
or Duspol rig or tacky taped up DIY globe poker thingie
can get you started good enough on a job to diagnose a faulty or suss RCD/GFCI
Yes it's best
to have a proper working RCD tester (or two) to test and check actual performance,
but if it's not there in a surprise situation, the other alternatives can get you started,
or at the least to verify a new replacement RCD/GFCI works