Use seed to randomize require order of test files

[halfdan]

Addresses https://groups.google.com/g/elixir-lang-core/c/yjs-zaFvZJc

Looking for feedback on implementation and testing strategy.

• Update docs for seed argument

[halfdan]

Had to be outside of Task.async to ensure seed was set when it's fetched in test.ex

[josevalim]

Those tests are rather expensive and they rely on implementation details-ish. I am thinking we could just skip them altogether? Other than that, the impl is great!

[halfdan]

Works for me! Let me remove the test.

[halfdan]

@josevalim I considered updating the docs for the --seed argument, but now I'm thinking that they are probably fine as is:

    * `--seed` - seeds the random number generator used to randomize the order of tests;
      `--seed 0` disables randomization so the tests in a single file will always be ran
      in the same order they were defined in

Even with --seed 0 there's no guarantee the modules are loaded in the same order given the parallelisation in how the modules are loaded.

[josevalim]

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