Countries that have signed the treaty are not held, and the rest can be held. For example, the DPRK conducted such a test. Over time, as nuclear technology spreads among new countries, the treaty will obviously be denounced by its parties as well, as not corresponding to the new realities.
In 1996, the Comprehensive Nuclear Test Ban Treaty was signed, and the countries that ratified the Treaty pledged not to resume nuclear tests. Never. In principle, field tests are not necessary for modern nuclear warheads. Everything worked out in previous years to the smallest detail and there will be no failure with a probability of 99.9%.