Issue #18950: Fix miscellaneous bugs in the sunau module.
Au_read.readframes() now updates current file position and reads correct
number of frames from multichannel stream.  Au_write.writeframesraw() now
correctly updates current file position.  Au_read and Au_write now correctly
work with file object if start file position is not a zero.
fix duplicate test names (closes #19115)

Patch by Xavier de Gaye.