The certification requirements only consider a dry runway (CS-25.125, FAR 25 similar). Wet runway performance is a multiple of dry performance, although many manufacturers will gather wet data as a check.
The requirements do not prohibit the use of reverse or any other device:-
Means other than wheel brakes may be used if that means
(i) Is safe and reliable;
(ii) Is used so that consistent results can be expected in service; and
(iii) Is such that exceptional skill is not required to control the aeroplane.
Few manufacturers elect to certificate their aircraft to this para, probably due to the complexities of proof of reliability, consistency of operation, and requiring separate data for engine failure:-
If any device is used that depends on the operation of any engine, and if the landing distance would be noticeably increased when a landing is made with that engine inoperative, the landing distance must be determined with that engine inoperative unless the use of compensating means will result in a landing distance not more than that with each engine operating.
Thus credit for reverse thrust during landing is generally not included in certificated landing performance.
EASA (CS-25 or alternative approved data) may allow reverse thrust for landing on contaminated runways, but as discussed previously the operator/commander has to consider the possibility of engine/reverse failure a hint that a larger distance factor should be used.
For info, because wet performance is only a multiple of dry data, based on an intermediate wet friction value, the safety margin in some wet conditions may not be as good as for dry conditions. In some circumstances wet landings may have very small safety margin due to the variability of wet conditions (not flooded/contaminated runway), poor runway surface, tyre condition, etc.