Add tests for the bit_struct! macro.

The macro crate, when built with `--test`,
now depends on itself (a version of itself built without `--test`).
This is a bit unusal, but allows testing a syntax extension.
I did not feel like making a new crate just for this,
and these tests do not belong in any of the other existing crates.
Merge pull request #3153 from SimonSapin/bit-struct-tests

Add tests for the bit_struct! macro. r=mbrubeck