testsuite: allow to check generated files

testsuite: allow to check generated files

This gives the test cases the ability to supply files that must be
checked after the test is run, rather than just checking stdout/stderr.

This is intended to be used with tools that generate files, like depmod.
It includes a poor's man implementation of a "check for differences in
files". Not really optimized, but it's simple enough and does what it
proposes to.