try to get the Pkg.test subprocess to detect sixel compat

[IanButterworth]

Currently is_sixel_supported() returns false within Pkg.test within a terminal that otherwise returns true, which is a shame because that's one of the most useful places for Sixel to run.

I think it's something to do with stdin not being forwarded properly by Pkg.test.
@oxinabox I recall you reported an issue like that in the past, but I couldn't find it in Pkg.jl issues.

Note stdin isn't forwarded here https://github.com/JuliaLang/Pkg.jl/blob/12c3eaf1a896edfc21bb935500bf905d42067a8a/src/Operations.jl#L2437

So in Sixel tests via Pkg.test we get

stdin = IOStream(<fd 8>)
stdout = Base.TTY(RawFD(10) open, 0 bytes waiting)
stderr = Base.TTY(RawFD(13) open, 0 bytes waiting)

[t-bltg]

I think it's something to do with stdin not being forwarded properly by Pkg.test.
@oxinabox I recall you reported an issue like that in the past, but I couldn't find it in Pkg.jl issues.

Is this JuliaLang/Pkg.jl#3185 ?