Modify test of synthetic tree (no longer provided)
Modify test of synthetic tree (no longer provided)

This now exceed the limit for tips in a tree, so attempts to fetch it
will now returns an HTTPError (400 Client Error: Bad Request) instead of
the previously expected ValueError.

OpenTreeOfLife/peyotl,mtholder/peyotl,OpenTreeOfLife/peyotl,mtholder/peyotl