lib-gvfs-helper: create helper script for protocol tests

The t5799-gvfs-helper.sh script is long and takes forever. This slows down
PR merges and the local development inner loop is a pain. Before
distributing the tests into a set of new test scripts by topic, extract
important helper methods that can be imported by the new scripts.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee