Auspice has no problem rendering (internal) nodes with a single
child (we use this in our SARS-CoV-2 build to encode travel
history). This updates the schema so that such builds pass.

Auspice (v2) JSONs are not allowed to have duplicate names (it causes
Auspice to crash). This modifies the vlidate functionality so that it
identifies this (previously there was no warning).

Adds minimal tests for validation of no duplicate names. Instead of
mocking up the tree JSON directly, these tests build a minimal BioPython
tree and use augur's own tree-to-JSON function to make the more verbose
JSON structure expected by the validation function.