LZ4 test uses incompressible test data, giving misleading results

[ComputerGuy4157]

The lz4 test (1.0.0) uses a Ubuntu ISO as its test file, this file is almost entirely incompressible as most of the data is inside a squashfs. lz4 skips over most of the file and gives misleading results that are 7-10x faster than when using lz4 against compressible data.

[svenha]

Some standard files used in compression comparisons might be a good choice, e.g. the silesia corpus, https://sun.aei.polsl.pl//~sdeor/index.php?page=silesia