Add integration tests for the `journald` source

[bruceg]

The journald source has no integration tests for ensuring it works with a running journalctl program. We should complete the coverage of testing this component by running against an actual journalctl program and against custom scripts that can exhibit key behaviors.

[jpds]

Separate to this, I have a few journalctl integrations tests over at: https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/vector/