tests: Fix incorrect stat.txt ignore when updating refs

tests: Fix incorrect stat.txt ignore when updating refs

ClassicTest was incorrectly ignoring stats.txt when updating reference
statistics. This was caused by ignore rules being applied too
aggressively when listing reference files. This changeset splits the
ignore rules into two different lists: 1) diff_ignore_files that lists
the files that shouldn't be diff:ed using the normal diff tool, and 2)
ref_ignore_files which lists files that should be ignored by the test
system.

Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>