Provide a test for output mode

[josephwright]

In issue 32 the point was made that using pdfTeX as an engine is not the same as PDF output. We don't currently have a test for the later. I'm not clear on whether we'll want one long-term (this depends on how output mode is selected), but we should perhaps have something at least in package mode. \mode_if_output_pdf:(TF) might fit with other names we currently use.

[blefloch]

I think that the \mode_ prefix is good, so I like the name. If we want to rename \luatex_if_engine:(TF), it could become \mode_if_engine_luatex:(TF), but that's another question.

[josephwright]

Collect potential use cases

[josephwright]

See also #233: at present the working name (in my head!) is something like \runtime_if_output_pdf:(TF)/\runtime_if_output_dvi:TF, dependent on final decision on module name here and what people think about the 'payload' naming.

[blefloch]

Fixed by Joseph some time ago: the output tests are \sys_if_output_dvi:TF and \sys_if_output_pdf:TF.