Tests: properly remove written files

`test_write_non_seekable_file()` was writing the file `delme.please` and not removing it after the test.

Instead of using the file name directly, I changed this to use the parametrized fixture `file_w` which automatically removes the temporary file after usage (and even after a failure).
I also used that in 2 other test cases.

Finally, I changed the options for opening a Python file object to `buffering=False`, otherwise some of our tests wouldn't succeed because the data is not written to disk even if the corresponding `SoundFile` object is closed during the test.