Differentiate between the presumed and actual file when dumping the AST to JSON

Currently, when dumping the AST to JSON, the presumed file is what is included
when dumping a source location. This patch changes the behavior to instead dump
the actual file, and only dump a presumed file name when it differs from the
actual file.

This also corrects an issue with the test script generator that would prevent
it from working on Windows due to file permissions issues.