GEOMESA-589 Create unit test using mock Accumulo to test mosaicing of rasters

Integrated code from old GeoMesaCoverageReaderTest into RasterTest. Generalized
and cleaned up code to use BoundingBoxes rather than just width and height. Rasters
are ingested into mock Accumulo and then queried out, and the byte array of the mosaiced
image before and after ingest are tested for equality.