[suppressions.yaml] Merge multiple files up the directory tree

[mikeharder]

When loading suppressions, merge all suppressions.yaml files found up the directory tree, from the fileUnderAnalysis to the root.

The merge is append-only, and the suppressions files are ordered from "closest to fileUnderAnalysis" to "closest to root". This is because tools may process suppressions as "first match wins", where this should match the suppressions file closest to the fileUnderAnalysis.

Also add end-to-end tests, generating files on disk dynamically at runtime (and of course run in parallel).