Previously, `mix run <tab>` would not expand into anything, even though the [`mix run`](https://hexdocs.pm/mix/Mix.Tasks.Run.html) task accepts filenames as inputs. This commit collapses the growing list of such cases (`test`, `format`, `run`) into one clause that autocompletes files.
Although `mix run` accepts several different `--flags`, the autocompletion isn't currently set up to deal with subcommands' specific options, so I'm punting on that part.
