tests/guests: Don't calculate checksums of generated images.

Turns out not to be useful, and takes a long time.

This reverts commit 0c605eb94f20cb08e4a858988e0da121af6f66cb.
tests/data: Remove abssymlink.

This file causes the test ISO to be rebuilt every time.

Instead of including it as a Makefile dependency, create it just
before making the ISO, which has the same effect.

A test in fish/ also used this file.  Change it so it doesn't need it.