Cleanup windows msi provider spec

More specs for https://github.com/opscode/chef/pull/2626

Added a spec that checks if the provider responds to shell_out, which unfortunately passes when running the specs because spec/support/platform_helpers.rb includes Mixin::ShellOut.

spec/support/platform_helpers.rb needs to be refactored to not do that, and ./spec/unit/mixin/shell_out_spec.rb needs to be cleaned up to not rely on Mixin::ShellOut being included globally.

cc @opscode/client-engineers 
Magic: https://github.com/opscode/chef/blob/master/spec/support/platform_helpers.rb#L4
I'm going to be refactoring that + the spec for Mixin::ShellOut