avocado.core.tree: Match whole key in filters.

When filtering nodes, use the whole key for comparing.

So if you have `/long` and `/longest` and you're filtering for `/long`, then it will match `/long` alone.

The previous behaviour was using the partial string match, so both `/long` and `/longest` were being returned, which is wrong.

Signed-off-by: Rudá Moura <rmoura@redhat.com>