Initially, we test that it can start (for T2621) and that it can be
asked to emulate a crash (for T2776).

Signed-off-by: Simon McVittie <smcv@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6988
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Add a dependency on the canterbury-core daemon. Until now, the canterbury-core tests were actually more like unit tests, linked directly to code that is used by the -core daemon, or were integration tests for canterbury-update-component-index. The new agents test is the first black-box integration test for the canterbury-core daemon.