$ mix test.watch

[warn] Could not start the file system monitor.

$ mix --version
Erlang/OTP 24 [erts-12.0.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Mix 1.12.2 (compiled with Erlang/OTP 24)

I could it to run by manually adding the executable flags:

$ chmod +x  ./deps/mix_test_watch/priv/zombie_killer
$ chmod +x ./deps/file_system/priv/mac_listener

Unluckily I do not now enough on the internals of the mix deps implementation and how to fix this.

I am running Mac OSX Big Sur 11.5.2, and I think it was working before.

$ cat mix.lock
%{
  "file_system": {:hex, :file_system, "0.2.10", "fb082005a9cd1711c05b5248710f8826b02d7d1784e7c3451f9c1231d4fc162d", [:mix], [], "hexpm", "41195edbfb562a593726eda3b3e8b103a309b733ad25f3d642ba49696bf715dc"},
  "mix_test_watch": {:hex, :mix_test_watch, "1.0.3", "63d5b21e9278abf519f359e6d59aed704ed3c72ec38be6ab22306ae5dc9a2e06", [:mix], [{:file_system, "~> 0.2.1 or ~> 0.3", [hex: :file_system, repo: "hexpm", optional: false]}], "hexpm", "7352e91952d9748fb4f8aebe0a60357cdaf4bd6d6c42b5139c78fbcda6a0d7a2"},
}