Test for taxonomy node hooks implementations

Attached a patch that improves and adds new tests to check that node hooks implementations are run correctly in taxonomy module.

Tests included:
- Test node-terms associations on node creation, with more than one vocabulary, and multiselect vocabulary.
- Test that node-terms relationships exist in the database.
- Test that taxonomy_node_load() populates $node->taxonomy correctly.
- Test taxonomy_node_view(). Check that the terms are displayed as links when the node is viewed.
- Test taxonomy_node_update() when editing the node with different terms.
- Test that taxonomy terms are copied to node when reverting a revision.
- Test taxonomy_node_delete_revision().
- Test taxonomy_node_delete().