Make the WDL file search path configurable.

[jdidion]

Right now, workflow_runner expects the WDL path to either be absolute, or to be relative to either the test context directory or the project root directory. Users have asked for the flexibility to support alternative project organizations, for example:

module
|_wdl
| |_myworkflow.wdl
|_tests
  |_test_workflow.py

This could be accomplished by defining a fixture (scope=function) that supplies the search path, that the user can override.