Always use the FileWriterGeneratorStrategy when running tests

[Ocramius]

As it currently stands, the test suite always uses eval() when running tests.

This is optimal for performance, but leads to a series of "WTF" moments when the user needs to actually debug what is going on in the generated code.

Instead, each test run should write proxy files inside a "build" directory, and require them from there. This way, should there be a failure, the failure report can be filed with the actual generated code.