Improving sanity checks

[franv314]

• Added hard errors for non UTF-8 or non printable output from the checker
• Uniformised grader attachements to grader.ext and never stub.ext
• Added warnings for solution checks of samples
• Removed check for consistent subtask ordering (imho, sanity checks should catch little mistakes, subtask definition and order is something high level enough that it makes no sense to fire warnings if tmr can't understand their order)

One annoying warning that is still present is that sometimes the I/O format of the samples differs from that of the true testcases and task-maker still creates warnings in this cases.
One possibility would be to create some tmr.toml file to specify things such as this one, or other unusualities in the task.