docs: add example of initializing the logger from a unit test

[pnkfelix]

Since I do not expect rust-lang/rfcs#1664 to be addressed in the short term, it would be a good idea to update the docs for env_logger to include an example usage on a few #[test] functions in addition to the current fn main.

(That is, I think the current docs leave a lot up to the intutition of the reader in terms of understanding how to generalize that one use of env_logger to other contexts.)

[dtolnay]

Looks like this was added to the readme in #56. It still needs to be added to env_logger's documentation.

[sfackler]

env_logger has moved out of tree.