When doing out-of-tree builds, abs_top_srcdir may well contain ".."
segments, and grub-syslinux2cfg canonicalises its --root argument. As a
result, the expansion of @abs_top_srcdir@ may not match what
It's somewhat difficult to portably canonicalize a path in shell, and
autoconf/automake don't offer any support for this. But there's a much
simpler option: copy the test data to a temporary directory and make
substitutions in the expected output file based on that.