Support GitHubActions logger as opt-in

[christopher-dG]

Using GitHubActions.jl.

I'm not actually sure how to implement this though, since the tests run in a child process. For Julia 1.3+, you could write to $JULIA_DEPOT_PATH/config/startup.jl and run Pkg.test(; julia_args=["--startup-file=yes"]).
To support older Julia versions, I think you'd have to write to the beginning of runtests.jl to set the logger.
Maybe I'm missing a simpler way, though.