`flush()` function implementation

Implements `flush()` function to cleanup the `FakeStream`.

Example : 
```c
test(myTest)
{
  FakeStream stream;
  stream.write(0x00);
  assertEqual(stream.bytesWritten().length(),1);
  stream.flush();
  assertEqual(stream.bytesWritten(), "");
  assertEqual(stream.bytesWritten().length(), 0);
}
```