When the `qztest` executable is built it links to` libquazip.dylib`, which has not yet had its lib id fixed by `install_name_tool`, i.e. it is just  `libquazip.dylib`. Since `qztest` is only for testing, I thought it best to put it in `libexec` instead of `bin`, where its linkage would have probably been fixed up.

Due to this, when running `qztest`, it can not find `libquazip.dylib` without a little help. Thought it was simpler to add the env var than add code to run `install_name_tool` on `qztest` to fix the link.