[fix] link zlib statically, with better test

[mriehl]

See #41 #38

Fixes #37

This is basically #38 with the tests encoded using cargo new semantics (no docker volume for the project)

[mriehl]

Here's the travis job: https://travis-ci.org/mriehl/rust-musl-builder/jobs/358272959
I hope I didn't break the travis config while refactoring, standing by..

[mriehl]

https://travis-ci.org/mriehl/rust-musl-builder/builds/358272958 still green

[mriehl]

Note that I tested the "file check" locally too. This was missing as pointed out by @bomgar because if the file doesn't exist, ldd also fails (with the same exit code).
There's a bit of duplication in the file path, but using a variable is such a fuckery in the multiline string (especially since I wanted to split it into path components because the test is slow as hell so getting tree like output is critical imho)

[bomgar]

@emk isn't this exactly what you wanted?

[emk]

Merged after overhauling the tests. Thank you!