Write automation to cleanup window.{variable}

[dmorejon]

js-test.js tests usually use window to set variables in order for eval() to work.
For example: window.files = files;
When we convert those functions to testharness.js, we need to clean up those variables. It doesn't affect the correctness of the transformation in most cases, but this would be nice since it would make the new test less confusing and more readable.

For now, cleanup can be done manually after transformation.