Shrink test image

The last (`IDAT`) chunk in this file starting at `0x5e265` reports to
have a length of `0x2000` bytes, but there are only `0x1D9B` bytes
left. Simply cutting the first `IDAT` chunk which starts at `0x31` and
also reports a length of `0x2000` at the same offset should produce the
same test results (while reducing the file size to 7.628 bytes).

Author: cmb69