... During certification, the airplane is flown at Maximum (Mach) Operating Limit Speed in level flight, then the nose is pitched down 7.5° for twenty seconds, followed by a 1.5g pull-up. The speed reached is V_D (by definition), and the aircraft controls are cleared for flutter on paper to 120% of this. You can check out the criterion at CS 25.335. CS 25 is available from the EASA WWW site under Certification.
Just to comment that in some (most? many? - can't speak for everyone) cases the 25.335 "manoeuvre" is, as per the suggestion in 25.335(b)(1) calculated rather than performed, in order to avoid flying an inherently hazardous manoeuvre. There are other requirements which do require flight demonstration, and to speeds approaching VD/MD, but not necessarily reaching them. 25.251 and the immediately following regulations either require flight to VDF/MDF or demonstration that this speed is not exceeded. Flight flutter testing must also, per 25.629(e), demonstrate flight at VDF/MDF - albeit with another caveat about use of analysis in lieu of test, although that is harder to achieve than for 25.335.