The method AbstractTreeViewer#internalExpandToLevel() used be a
recursively called method (by implementation and specification). As a
protected method, it is part of the API.
With a recent change, the method was changed to delegate to a different
method, removing it's recursive property. Consumers that have created
subtypes of the AbstractTreeViewer and rely the recursive behavior of
that method by overwriting it face a regression by that change.

This change restores the existing, recursive behavior of that method
while still keeping the reuse of the implementation across other use
cases. A test case ensuring the contractual recursive execution is
added.