If I look around i don't see that documented. 
https://github.com/tianocore/edk2#code-contributions
https://github.com/tianocore/tianocore.github.io/wiki/Code-Contributions
https://github.com/tianocore/tianocore.github.io/wiki/How-To-Contribute
https://github.com/tianocore/tianocore.github.io/wiki/How-to-Become-a-Contributor

In Laszlo's massive write up that is very detailed for the contribution process there is nothing calling out that the contributor must test on any given platform.  
He does however suggest that maybe as a maintainer you should test it but again no indication of how/what/where.  
https://github.com/tianocore/tianocore.github.io/wiki/Laszlo's-unkempt-git-guide-for-edk2-contributors-and-maintainers#maint-13

In all the teams i have worked with over the years no one has used OVMF or emulator package as the verification for an edk2 patch.  Maybe that is just my experience.  

Overall my point is I don't think this is clear to the community and I believe that adding this requirement without actually building the full infrastructure online will be detrimental to the contribution process.  And if building the infrastructure online then build it to scale to N platforms to cover more of the edk2 community and not just those in the edk2 tree.

Thanks
Sean