[DCP] Test Design.writeCheckpoint() when using existing EDIF

[eddieh-xlnx]

An upcoming release will support the ability to write out the EDIF netlist prior to detaching, without compromising the ability to write out a DCP checkpoint:

EDIFTools.writeEDIFFile(design.getNetlist(), <EDIF path>, ...);
design.detachNetlist();
design.writeCheckpoint(<DCP path>, <EDIF path>, ...);