Simplify use of CompositorData::find_layer_with_pipeline_and_layer_id

Instead of always matching on the root layer first, have a Compositor
method which abstracts this away, simplifying all the call sites of this
CompositorData helper.
Reftest harness walks directory and appends tests for all manifest files
it finds. This is a first step towards importing some of the css-wg reftests.
Merge pull request #3117 from mrobinson/simplify-find_layer_with_pipeline_and_layer_id

Simplify use of CompositorData::find_layer_with_pipeline_and_layer_id
Merge pull request #3118 from glennw/reftest-scan

Reftest harness walks directory and appends tests for all manifest files