Improve zombie_killer compatability

Problem:
- Combination of zombie_killer and test command quotes are causing
  the unit tests to not run

Solution:
- Remove use of exec in sh script because it is being used incorrectly
- Change command quoting to match what is conventionally used with sh -c

lib/mix_test_watch/port_runner/port_runner.ex

@@ -17,7 +17,7 @@ defmodule MixTestWatch.PortRunner do
 
       _ ->
         Path.join(:code.priv_dir(:mix_test_watch), "zombie_killer")
-        |> System.cmd(["sh", "-c", command], into: IO.stream(:stdio, :line))
+        |> System.cmd(["sh", "-c", "'" <> command <> "'"], into: IO.stream(:stdio, :line))
     end
 
     :ok
@@ -40,8 +40,8 @@ defmodule MixTestWatch.PortRunner do
 
     ansi =
       case Enum.member?(config.cli_args, "--no-start") do
-        true -> "run --no-start -e 'Application.put_env(:elixir, :ansi_enabled, true);'"
-        false -> "run -e 'Application.put_env(:elixir, :ansi_enabled, true);'"
+        true -> "run --no-start -e \"Application.put_env(:elixir, :ansi_enabled, true);\""
+        false -> "run -e \"Application.put_env(:elixir, :ansi_enabled, true);\""
       end
 
     [config.cli_executable, "do", ansi <> ",", task, args]

priv/zombie_killer

@@ -1,16 +1,16 @@
 #!/bin/sh
 
 # Start the program in the background
-exec "$@" &
+eval "$@" &
 pid1=$!
 
 # Silence warnings from here on
 exec >/dev/null 2>&1
 
 # Read from stdin in the background and
 # kill running program when stdin closes
-exec 0<&0 $(
-  while read; do :; done
+0<&0 $(
+  while read UNUSED ; do :; done
   kill -KILL $pid1
 ) &
 pid2=$!