avocado.core.tree: Introduce apply_filters()

Introduce function apply_filters(), which will prune the tree, by
using filters with rules for filter only and filter out nodes.

So it works for the test runner and for the multiplex plugin,
or anything that uses the treenode structure with filters.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
multiplexer_unittest: Test everything with apply_filters().

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.core.tree: Don't yield root leaf.

When iterating for leaves, in the case that leaf is the root node,
don't yield it, just ignore.

Signed-off-by: Ruda Moura <rmoura@redhat.com>