harness: add gst_harness_take_all_data() + _take_all_data_as_{bytes,buffer}()

harness: add gst_harness_take_all_data() + _take_all_data_as_{bytes,buffer}()

Convenience function to just grab all pending data
from the harness, e.g. if we just want to check if
it matches what we expect and we don't care about
the chunking or buffer metadata.

Based on patch by: Havard Graff <havard.graff@gmail.com>