test: create file then append data

[zpg6]

Initiated from #172 - it was recommended to add a test case to further investigate my issue.

The added test confirms that opening a file as ReadWriteCreateOrAppend and then writing to it several times works properly.

[zpg6]

@thejpster - it seems to work fine. But how to better test that the underlying files have changed? Because when I run the same on a SPI device the file sizes increase but there's only an empty 0KB file on the SD card when I plug into Windows.

UPDATE: solution was to make sure you file.flush(). For a device like mine where power could turn off at any time, I just flush after each write and its fine.

[thejpster]

Yes, to avoid excessive wear on solid-state drives, we don't update the directory entry until you close the file or you flush the file.

[zpg6]

@thejpster anything you'd like to see changed or added to make this a useful test? Otherwise can close.

[thejpster]

I think flushing is already covered by

embedded-sdmmc-rs/tests/write_file.rs

Line 59 in 6ebe113

fn flush_file() {

.