syslinux_test: Fix out-of-tree build handling

syslinux_test: Fix out-of-tree build handling

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
grub-syslinux2cfg produces.

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.

Forwarded: http://lists.gnu.org/archive/html/grub-devel/2016-09/msg00013.html
Last-Update: 2016-09-19

Patch-Name: syslinux-test-out-of-tree.patch