Add retrieval testing for InlineSchemaTree

For now, one case works.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
JsonPointer: add a .relativize() function, test that it works

This will return the subpointer of the target only if the current pointer can be
considered a parent, otherwise the target itself.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>