Thanks, adding that to config/test.exs & chromedriver to PATH changed the error message at least!!
$ mix test
Compiling 18 files (.ex)
Generated chatter app
** (MatchError) no match of right hand side value: {:error, {:wallaby, {{:shutdown, {:failed_to_start_child, Wallaby.Experimental.Chrome, {:shutdown, {:failed_to_start_child, Wallaby.Experimental.Chrome.Chromedriver, {:eacces, [{:erlang, :open_port, [{:spawn_executable, "c:/projects/chatter/_build/test/lib/wallaby/priv/run_command.sh"}, [:binary, :stream, :use_stdio, :stderr_to_stdout, :exit_status, {:args, ["c:/tools/chromedriver/chromedriver.exe", "--log-level=OFF", "--port=57042"]}]], [file: 'erlang.erl', line: 2215]}, {Wallaby.Experimental.Chrome.Chromedriver, :init, 1, [file: 'lib/wallaby/experimental/chrome/chromedriver.ex', line: 22]}, {:gen_server, :init_it, 2, [file: 'gen_server.erl', line: 374]}, {:gen_server,
:init_it, 6, [file: 'gen_server.erl', line: 342]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 249]}]}}}}}, {Wallaby, :start, [:normal, []]}}}}
test/test_helper.exs:4: (file)
(elixir 1.10.2) lib/code.ex:917: Code.require_file/2
(elixir 1.10.2) lib/enum.ex:783: Enum."-each/2-lists^foreach/1-0-"/2
(elixir 1.10.2) lib/enum.ex:783: Enum.each/2