Is it, really? If the whole point of the library is dealing with binary files, how are you even going to have automated tests of the library?

The scary thing is that there is people still using autotools, or any other hyper-complicated build system in which this is easy to hide because who the hell cares about learning about Makefiles, autoconf, automake, M4 and shell scripting at once to compile a few C files. I think hiding this in any other build system would have been definitely harder. Check this mess:

  dnl Define somedir_c_make.
  [$1]_c_make=`printf '%s\n' "$[$1]_c" | sed -e "$gl_sed_escape_for_make_1" -e "$gl_sed_escape_for_make_2" | tr -d "$gl_tr_cr"`
  dnl Use the substituted somedir variable, when possible, so that the user
  dnl may adjust somedir a posteriori when there are no special characters.
  if test "$[$1]_c_make" = '\"'"${gl_final_[$1]}"'\"'; then
    [$1]_c_make='\"$([$1])\"'
  fi
  if test "x$gl_am_configmake" != "x"; then
    gl_[$1]_config='sed \"r\n\" $gl_am_configmake | eval $gl_path_map | $gl_[$1]_prefix -d 2>/dev/null'
  else
    gl_[$1]_config=''
  fi

It's not uncommon to keep example bad data around for regression to run against, and I imagine that's not the only example in a compression library, but I'd definitely consider that a level of testing above unittests, and would not include it in the main repo. Tests that verify behavior at run time, either when interacting with the user, integrating with other software or services, or after being packaged, belong elsewhere. In summary, this is lazy.

As mentioned, binary test files makes sense for this utility. In the future though, there should be expected to demonstrate how and why the binary files were constructed in this way, kinda like how encryption algorithms explain how they derived any arbitrary or magic numbers. This would bring more trust and transparency to these files without having to eliminate them.

Yep, I consider it a failure of the build/dev pipeline.