tests/cachekey: Test cache keys are independent of target elements

Description

When @BenjaminSchubert implemented a new dependency sorting algorithm, it passed the unit tests. However, under this algorithm the cache keys of an element were depended on the target element(s) when invoking bst.

Changes proposed in this merge request:

• Add a test to help catch any variation in cache keys which is dependent on the target elements and the order in which targets are specified. This test fails for the branch https://gitlab.com/BuildStream/buildstream/commits/bschubert/optimize-sort