License of Rust logo test file

[robo9k]

It seems that the inherited data/tests/rust-logo-128x128-blk.png is from e.g. rust-lang/rust-www which seems to be licensed as CC-BY (see rust-lang/rust#11562).

Given the requirements of CC-BY it might be desirable to replace rust-logo-128x128-blk.png with another test file.