Add a new helper function to test cache key stability
This adds a new buildstream.testing.helpers
module that will
contain methods to simplify commonly used pattern in tests of plugins.
The first method to be extracted there is check_cache_key_stability
.
This will allow plugins to test more easily that their cache keys
remain stable.
NOTE: This still needs to be included in the documentation, but we should probably create an entire 'plugin author' section.
Edited by Benjamin Schubert