Add YAM cache testing to yaml tests and further YAML cache refinements

Description

This is based upon !787 (closed), combines yaml cache changes added under !787 (closed) with requested test case in yaml test.

Changes proposed in this merge request:

• Change 1 - Add Yaml cache changes as per !787 (closed)
• Change 2 - Add test case for list composition to yaml test.

See !787 (closed) (#466 (closed))

• Now moved back into WIP until issues are resolved.