xzgrep: exit 0 when at least one file matches.

Mimic the original grep behavior and return exit_success when
at least one xz compressed file matches given pattern.

Original bugreport:
https://bugzilla.redhat.com/show_bug.cgi?id=1108085

Thanks to Pavel Raiskup for the patch.
xzgrep: Add a test for the previous fix.

This is a simplified version of Pavel Raiskup's
original patch.
xzgrep: Improve the test script.

Now it should be close to the functionality of the original
version by Pavel Raiskup.
xzgrep: List xzgrep_expected_output in tests/Makefile.am.