tests/sources/keytest: Ensure element state is updated as expected
This contains two changes:
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tests/sources/keytest.py: Ensure that the element state before/after BuildStream operations is updated as expected. Previously we were just checking that the command succeeded.
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tests/sources/project_key_test/plugins/sources/key-test.py: Update the dummy plugin that's used for tests, such that it conforms to the expectations of a BuildStream source class.
Fixes #1179 (closed).