test: compression: check with trailing garbage in input

The Linux kernel appends 4 bytes to the end of compressed kernel Images
containing the uncompressed image size. They are used to make
self-decompressing Images easier. However for archs that don't support
self-decompression, like ARM64, U-Boot must be able to decompress the
image with the garbage data.

The existing decompressors already support this. This unit test was
added while working on zstd support as upstream zstd will error if there
is garbage data in the input buffer, and special care was needed to
support this.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Reviewed-by: Simon Glass <sjg@chromium.org>