ctest_submit: Add support for a "Done" part
Teach CTest to submit Done.xml. Submission of this file indicates to CDash that a build is complete and no more files will be uploaded. It contains the build id returned by CDash and the current time. This file is submitted last for a given build when using the `ctest_submit()` command. If submitting by PARTS, use `ctest_submit(PARTS Done)`.