Benchmarking: Fashion a load test to trigger journaling jbd2 lock

In our benchmarking test suite, we should separate out a standard test that can be used to compare different configurations eg: with & without reftables, with & without overlayfs, etc.

For load tests where we want to find and expose a specific bottleneck, we should have separate tests for each bottleneck.

For example, we know that the jbd2 journaling call is a bottleneck. Let's fashion a specific test to trigger this bottleneck.