Print to stderr shows up in notebook

[fonsp]

Since fonsp/Pluto.jl#1957, the fallback printing also shows up in Pluto :(

@dralletje Why was this added? Just debugging?

PlutoTest.jl/src/notebook.jl

Lines 1684 to 1686 in 299765f

	try
	Test.do_test(julia_test_result, $(orig_expr))
	catch; end

[dralletje]

Oof. It was for compatibility with ] test, didn't think it would hurt if we'd show it in Pluto 🤦‍♂️ so I'd still keep it, but "just" not when we're in Pluto..