Auto merge of #6327 - ehuss:add_plugin_deps-tests, r=alexcrichton
Fix add_plugin_deps-related tests.
These tests were modified in #3974 in such a way that they stopped testing the `add_plugin_deps` code path. The tests can't be directly reverted because #3651 changed it so that dylib paths must be within the root output directory. I compromised by just copying the dylib into `$OUT_DIR`.
Closes #6318.