Prepare tests runner for elm-test-rs

[mpizenberg]

Improvements of the tests runner for elm.

• Use a 2-stages Docker build for a lighter runner container
• Create ~/.elm cache in the build stage, based on the templates/elm.json file of the elm track
• Use elm-test-rs instead of elm-test (not finalized yet, waiting on --report exercism option in the making)

[mpizenberg]

I'm struggling with how to make the elm compiler not hang. Normally, if the elm.json did not change since last time we compile, the compiler should assume that dependencies are good and immediately compile stuff. But if elm.json is updated, the elm compiler checks the dependencies, and for that it does some weird network interaction, resulting in 5s of hanging when there is no network.

So I put much care in trying to keep files timestamps, by using tar to create and extract an archive containing .elm, elm.json and elm-stuff. Yet, when running bin/run-in-docker.sh, we spend more than 5s in the elm compiler where this should take less then 1s. This (tar) is obviously not enough to prevent the network hang of the elm compiler, and I'd like to figure out how to fix it.

[mpizenberg]

I've added tinyproxy to set a small proxy configured to fail all http requests. This effectively makes the elm compiler immediately switch to its offline mode and prevents the 5s hang :)

I've also added a line to "un-skip" all the skipped tests in the tests/Tests.elm file.

[mpizenberg]

Alright I think this is ready for review. Here is a recap of the changes:

• Now using a 2-stage Docker build, making the final container smaller, from 232MB down to 162MB.
• I've added a proxy server (tinyproxy) configured to fail all requests. This effectively forces the elm compiler to immediately switch to offline mode and prevents a weird 5 seconds hang otherwise observed.
• The elm cache inside ~/.elm/ is now generated by the build step of the container, based on the elm.json template file of the exercism/elm repository so it is the same as the one used to run tests in that repo. Previously we had manually pre-package a ~/.elm/ dir and we used to download it during the container build.
• We are using jq now to generate the results.json in case the tests runner errors. This is much cleaner than the previous shell script.
• The tests runner used is now elm-test-rs instead of elm-test. This brings numerous advantages such as:
  • It has a --report exercism option so we don't need the post-processing python script anymore. This removes both the file and the need to have python installed in the container.
  • It is faster than elm-test. For a small project like those that will run for exercises, it brings the timing down from roughly 1.8s to roughly 0.3s. Combined with the fact that we shaved 5s thanks to the proxy trick, this new tests runner feels instantaneous compared to the old one.
  • It is a portable executable so we are now free from npm. Both elm and elm-test-rs can be installed by downloading an executable. This means I removed the package.json which is not needed anymore.

[mpizenberg]

PS: the windows script run-in-docker.ps1 needs a small update to match the few changes in run-in-docker.sh.

[mpizenberg]

Excited to be able to test this update of the tests runner once #10 (which is also ready for merge) is merged :)