d/t/unit-test: copy test-suite.log to AUTOPKGTEST_ARTIFACTS

Now we can access the log by downloading the artifacts directly from debci or salsa-ci, no need to re-run locally to get it using a shell after the failure.

This is very helpful for debugging dependency updates for example this one. I had to download liboauth2 source and run autopkgtest locally with --shell-fail to inspect test-suite.log. Now one can simply browse the artifacts to get it.