Add --only task_id={number} to useful mix test options

[jrgtt]

Hello, this is my first time trying to contribute here. I have read HELLO.md and CONTRIBUTING.md, and I hope this PR meets the criteria.

I came across this option while inspecting the help guide of mix test and I have found it quite fitting for the way I approach learning exercises in the platform. Which is:

1. Read the instructions
2. Pull the code with the CLI command
3. Read the next instruction (task) and try to implement it locally
4. Run mix test --only task_id:{instruction_number}
5. Fix it if needed
6. Repeat from step 3 for the next instruction
7. After the last instruction is completed, run mix test to check everything

Before finding out about this option I was running all tests together but that created a lot of noise, now I can tackle each instruction one after the other. That's why I think sharing it with other users who prefer working locally on the exercises could be useful.

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[angelikatyborska]

Thanks a lot! That's a very useful addition 💜