Add tests for juliaup-init installation script

Once we get movement on https://github.com/JuliaLang/juliaup/pull/143, we should add some tests to make sure that julia-init works on a wide selection of platforms, and that new releases to https://github.com/JuliaLang/juliaup/releases doesn't somehow break the installation scripts.

Start by reviewing pull request 143 and the current juliaup-init installation script. Determine how tests can cover multiple platforms and releases without relying on undocumented assumptions. The work is done when installation regressions caused by supported platforms or new releases are detected automatically.

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