Make running the tests locally fun

[t4lz]

Our test suite is currently not very friendly to run locally. You have to build a lot of little test applications.
This leads to most of us just relying on GitHub actions to run tests, which probably slows down our work.

Building the tests apps should be automated, and the same automation should be used to build the test apps in the CI.
That way our ci.yaml would become a lot more concise, and also the automated building would be guaranteed to remain up-to-date.

Building the test apps could maybe be part of running the tests, like in:

mirrord/mirrord/layer/tests/http_mirroring.rs

Line 19 in 94d50e0

fn build_go_app() {

Or maybe it's possible to do it in a build script that runs when we execute the tests if any files changed.
At the very least, we could have a bash script that builds all the test apps.

After automating building the test apps, if that step is not transparent, we should document it everywhere a new contributor might look for it.

[aviramha]

I suggest using the xtask pattern / https://github.com/jondot/xtaskops