Add libc direct tests for (get/set/unset)env functions

[eduardorittner]

This PR adds tests for the getenv, setenv and unsetenv functions directly through the libc api. Related to #3179

[rustbot]

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[RalfJung]

Thanks! I left some minor comments.

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[RalfJung]

@rustbot ready

[RalfJung]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[eduardorittner]

@rustbot ready

[RalfJung]

This looks great, thanks! Please squash the commits. You can squash manually if there are multiple independent commits you want to preserve, or use ./miri squash (make sure to pick a suitable commit message). Then write @rustbot ready after you force-pushed the squashed PR.

@rustbot author

[eduardorittner]

@rustbot ready