partial photo/image tests without Img extension?

I notice that both Tcl::Tk and Tcl::pTk have some tests that currently =
require the Img extension to handle image formats not included in =
Tcl/Tk. Would it be desirable to still test the formats that are =
available (gif; png on Tk 8.6), or is the test more about exercising the =
capabilities in Img? Is it even possible to plan a certain number of =
tests and then skip a subset of them?

This is low/=E2=80=9Cwishlist=E2=80=9D priority; I ask only because at =
the moment MacPorts doesn=E2=80=99t have the Img extension.