Expand capability of testBuilder

[natebosch]

• Add back onLog argument and dep on package:logging since there is a
  user of this feature in the old testPhases that needs to be migrated
• Only do any checking of outputs when the outputs argument is
  non-null rather than expect empty outpus on null
• Don't double up on the check against actual assets. This was a
  holdover from the version where we were checking both written assets
  and the results object
• Add in an isInput callback as an alternative to generateFor since
  this is sometimes an easier way to express the constraints (ie when
  all assets in the root package are inputs)
• Add back the writer argument to allow stubbing behavior or doing
  more complex validation. Take in an AssetWriter and spy on it rather
  than require an InMemoryAssetWriter
• Add a method to make new assets available through the
  InMemoryAssetWriter rather than require they are first written through
  the writer and sharing the cache

[kevmoo]