As briefly discussed in [#5174], regtests are not quite as useful when making changes with a target of the SVG backend. Further, there have been regressions that only affect this backend (such as [#3778]). Allowing regtests to use multiple backends would prevent changes that are harmful to a specific backend, and would allow greater flexibility to the regtests to be effectively used.
Whoops, this is the same bug as [#2230], but its patch was abandoned, thus closed, it seems.
Related
Issues: #2230