test: make test runner generic
test: refactor multipart testing

* Specify object passed to Dicer constructor
* Check preamble
* Show actual vs expected for preambles and part headers
test: add multipart test to verify preamble header parsing
dicer: move streamsearch instance 'info' handler to prototype method, add setBoundary()

setBoundary() is for when parsing a preamble header, where the boundary would be found