Since TwnMainPage is only used by translatewiki.net, which uses PHP, there is no added value running HHVM tests. In fact, it decreases value, as the parent task indicates, by limiting what we can do.
It seems extension-quibble should be replaced with either extension-quibble-composer-nohhvm or extension-quibble-php72-plus. I am not fully sure what are the differences. Former seems better, but what exactly does the composer part in it mean?
For historical reasons CI default to use mediawiki/vendor.git to ship the composer dependencies.
extension-quibble-php72-plus uses mediawiki/vendor.git and does not use php7.0 / php7.1
extension-quibble-composer-nohhvm uses composer install instead.
Or we craft yet another template that uses composer install and only uses php 7.2. I guess that depends on which version of php you are using on translatewiki.net.