As described in David Shuetz’ 2011 BlackHat conference talk Inside Apple's MDM Black Box the easiest way, perhaps, to obtain an APNS MDM push certificate for testing is to export it from your macOS Keychain after installing macOS Server's Profile Manager MDM server. Please see page 7 of Shuetz white paper PDF (alt. PDF link) available on the Black Hat 2011 talk page for details.
Of course it is recommended that you pay for a subscription to an Apple's Developer program to obtain a proper certificate for any production MDM server.
Please note: the mdmcert.download project is not officially a part of the MicroMDM open source MDM project. Please do not file an issue on the MicroMDM project page for any problems with mdmcert.download. You are, however, welcome to come join the community on the MacAdmins slack (in the #micromdm channel) to discuss any issues with the community. Thank you.