Fix race condition in OutputCapture

[schmidti159]

There is a race condition in OutputCapture used in the OutputCaptureExtension that could lead to flaky test results.

This race could occur because of the existing cache in OutputCapture: When data was written, while it was read, any following reading of data might miss the last output. The cache was not invalidated correctly in this case.

// Thread 1 in get()
String result = existing.get();        // Gets null
if (result == null) {
    result = build(filter);            // Builds string WITHOUT synchronization
    // <- RACE WINDOW: Thread 2 can clear cache here
    existing.compareAndSet(null, result); // Succeeds, caches stale data
}

We are now versioning the cache entries to make sure this can't happen anymore. When invalidating the cache we also increase the version.