Fix test specs

[cfurrow]

Fixes spec failures that I found when pulling down master on macOS, perhaps other envs too.

1. Use Dir.tmpdir instead of the rspec-bash's create_stubbed_env #dir method, which does not seem to exist anymore?
2. There seemed to be an invalid stubbing of the executor methods. The code uses DockerSync::Environment.system, but the spec expected it to be DockerSync::Dependencies::Docker::Driver.system
3. Added more OS conditions for detecting .mac?, since .linux? did not capture it.

 Failure/Error: FileUtils.cp_r([host_app_template, config_file_template], env.dir)

      NoMethodError:
        undefined method `dir' for #<Rspec::Bash::StubbedEnv:0x00007fb0eeaf9988>
      Shared Example Group: "a synchronized directory" called from ./spec/integration/native-osx_strategy_spec.rb:43
      # ./spec/integration/native-osx_strategy_spec.rb:24:in `block (3 levels) in <top (required)>'

 CommandExecutionMock::CommandExecutionNotAllowed:
        Command execution is not allowed. Please stub the following call: __system_without_any_instance__("pgrep -q com.docker.hyperkit")
      # ./spec/helpers/command_mocking_helpers.rb:36:in `block in stub_command_executor'
      # ./lib/docker-sync/environment.rb:22:in `system'

[EugenMayer]

Thank you a lot for working on this!

[cfurrow]

No problem! I was going to look into updating this gem for Ruby 3.x+, but ran into some other issues along the way with Backticks/docker-compose and their own Ruby 3.x+ issues, so I have shelved that work for now 😀