nixosTests.forgejo: test backup/dump service

This should allow us to catch issues regarding that in the future.

nixos/gitea had an issue with the dump service recently, which didn't
affect us, fortunately.
But to be fair, it only affected non-default-y setups.
Not something we are able to catch in the current, rather simple, config
of our test.
Still, I see a lot of value adding this new subtest to our test suite.

Anyhow, this patch also exposes the resulting tarball as test (build)
output, which is a nice addition IMHO, as it allows some sort of
external sanity-check, if needed, without running the test interactive.

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