Test against the XZ test suite

[Shnatsel]

The XZ utils source distribution at https://tukaani.org/xz/ contains 63 small files exercising various features of the format. The test files are placed in the public domain and there is a description of the expected behavior for each one.

It would be nice to test lzma-rs against those files - it would have caught #32, for example.

[Shnatsel]

See also #20

[Shnatsel]

xz2 crate already does this, its test harness can be used as reference: https://github.com/alexcrichton/xz2-rs/blob/master/tests/xz.rs

[gendx]

Good idea! Note that only the basic filters are implemented for now, so some of the "good" files will currently be rejected.

[gendx]

Note: these files can also be found in https://github.com/xz-mirror/xz/tree/master/tests/files.