How is this being received by the international community and North Korea watchers?
Could this be a sign of Pyongyang showing sincerity towards the summits with South Korea and the U.S.?

Well, I'd definitely have to say that views are starkly divided,... but some do think that the North has vowed to take some of the more specific steps that Washington would like to see in the process of denuclearization -- by not only declaring an end to nuclear and missile provocations... but also mentioning the dismantlement of the nuclear test site.
And this move came without any predetermined reward -- unlike in 2008 when Pyongyang destroyed a cooling tower for a nuclear reactor in Yongbyon... in return for the U.S. removing North Korea from its list of state sponsors of terrorism.
The North also attached no conditions to the moratorium on tests,... unlike in early March... when the North said it wouldn't carry out provocations (quote) "so long as talks continue."