[dvsim/testplanner] Allow duplicate testpoint names so long as they are in separate files

Allow DUT and its imported testplans to have the same testpoint name as long as they are in separate files.
* The list of written tests are appended from both files.
* The descriptions are merged - it's up to the user to ensure that it is still meaningful after the merge.
* Conflicting milestones are flagged as an error.

This future work item is also tracked at the bottom of `util/dvsim/doc/testplanner.md` (as of commit 86169d0)

Start with the future-work item at the bottom of util/dvsim/doc/testplanner.md and inspect the dvsim/testplanner implementation it references. Done means duplicate testpoint names are accepted only across separate files, written tests and descriptions are merged, and conflicting milestones produce an error.

Written by the indexing model from the issue text.