Fix tests to be portable to Windows

So the course of trying to actually add tests for https://github.com/gotwarlost/istanbul/pull/219 I found that I was unable to run the test suite on a Windows machine. This fixes the following issues when running on Windows:

1. The `npm test` command was missing the binary to run the file with, which is necessary for Windows.
2. There were a bunch of tests that used file paths and compared strings to file paths, but they assumed the path separator was a forward slash, though it is a backslash on Windows.
3. Due to a really crappy bug in Node.js on Windows when the process's output is piped (which is the case when calling with `spawn`), output from the child Node.js process will get cut off is the event loop is stopped before all the pending writes has completed. This caused lots of output-checking related tests to fail.