Add ImageFile _get_fp_length

[radarhere]

• After Fix heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO #10224, reduces some duplicate test code
• After Bound retained TIFF tag data by stream size #10225, adds _get_fp_length() methods to ImageFile, to move code for checking file handle length into a common place
• After Validate TIFF tag values against libtiff's own field definition before TIFFSetField #10227 changed TIFF tag type detection, remove a comment that 'Types should not be changed for backwards compatibility', since we just did that.