Make variables and functions in parent scope available within `@trixi_testset`

[benegee]

This came up while working on #6 .

It would be nice if we could write

f(x) = ...
@trixi_testset "" begin
    #use f
end

as can be done with Test.jl's @testset. This is currently not possible with @trixi_testset because we wrap everything inside a dedicated module TrixiTestModule

Should we add syntax like $EXAMPLES_DIR to look up things in the parent, instead of special casing?

This is how BenchmarkTools does it https://github.com/JuliaCI/BenchmarkTools.jl/blob/cddd794ba336763be6831cfa076d27df96ef893c/src/execution.jl#L361-L380

Originally posted by @vchuravy in #6 (comment)