How to link testdata

[sffc]

#306 includes the testdata as:

    let provider = FsDataProvider::try_new("../../resources/testdata/data/json")
        .expect("Loading file from testdata directory");

The crate documentation says:

let data_provider = icu_testdata::get_provider();

Decide what to use in tests and in examples.

[zbraniecki]

I decided to use testdata everywhere in code, and document the use of FsDataProvider in docs, tutorials and wiki.