Replace sf.SoundFile with sf.open in tests

This gives one more line of coverage ...
Make data_mono a column vector
Use read() function instead of read() method in some tests

This adds 9 lines of coverage.
Change file_stereo_w to file_w and file_stereo_rw_new to file_rw_new
Add a test for the write function

This leads to 5 more lines of coverage.
Use read() function instead of method for mono tests
Move all seek() tests into one section
Combine a few trivial test functions to reduce test runtime