tests: take exit codes into account

tests: take exit codes into account

In its current form the testsuite isn't suitable for running
fuzz targets because it ignores exit codes and relies solely
on diffs (that unfortunately aren't helpful because the nondeterministic
nature of fuzz targets makes it kind of hard to specify expected output
in advance). This patch is supposed to address the "exit code" issue for now.

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>