This removes the need for checking test only environs in the bin script.

This removes the need for a separate platform.js file and test environ.

This refactors the `src/rimraf-*` files to use `fixEPERM` and `ignoreENOENT` everywhere possible. The biggest churn here is updating the test files to correctly mock the necessary FS functions that are now spread across multiple files.

PR-URL: #322
Credit: @lukekarrys
Close: #322
Reviewed-by: @isaacs

PR-URL: #284
Credit: @SukkaW, @pmmmwh, @lukekarrys
Close: #284
Close: #314
Reviewed-by: @isaacs