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)`.
Status Job ID Name Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager