Improve the `PDFDataRangeTransport` unit-tests

- Add a new test using only streaming, since that was missing and the lack of which most likely contributed to previous bugs in the `PDFDataRangeTransport` implementation (see PR 10675 and 20634).

 - Improve the "ranges and streaming" test, to utilize both ranges *and* streaming properly, since the way it was written seemed somewhat unrealistic given how data will normally arrive when `PDFDataRangeTransport` is being used.

 - Provide more `initialData`, in relevant tests, since a length smaller than `rangeChunkSize` seem pretty pointless.

 - Test the `contentDispositionFilename`, and `contentLength`, handling in the `PDFDataRangeTransport` implementation.