Make $test_nowarn_mod available to downstream packages

[sloede]

          Can we somehow export this as well when importing TrixiBase from a testing environment? Then we can avoid duplicating this function from Trixi.

Originally posted by @efaulhaber in #5 (comment)

[ranocha]

My suggestion would be to call this macro @trixi_test_nowarn and include it in a package TrixiTest.jl.

[sloede]

Sounds good to me. I'd just like to gather some experience with (the additional management overhead of) TrixiBase.jl first, before adding more upstream packages.

However, if someone else is willing to set up TrixiTest.jl right now, I wouldn't be opposed either.