Always run `chef-client` via `ruby PATH/TO/chef-client` in tests.

Fixes CHEF-4914

Running `chef-client` directly can result in a "system" `chef-client`
being run (e.g., an omnibus install of chef-client). When testing on
different ruby versions, this can cause segfaults because bundler sets
environment variables that cause C extension gems compiled against a
different ruby ABI to get loaded. For example, a JSON gem compiled
against ruby 1.9 can be loaded into ruby 2.0.