Add unit-test for batch processor to validate force flush get's called.

[hdost]

Can we also add a unit-test for batch processor, with mock-exporter, to validate that the exporter's force flush get's called.

Originally posted by @lalitb in #2276 (comment)

[blue-orange-yellow]

@cijothomas
Hi, I’m interested in contributing to this issue. This would be my first contribution to this project, so any advice or guidance would be appreciated.

Before adding tests, do we first need to add a force_flush method to the LogExporter trait, something like this?

// opentelemetry-sdk/src/logs/export.rs

pub trait LogExporter: Send + Sync + Debug {
    ...
    /// Flushes any log data
    fn force_flush(&self) -> OTelSdkResult {
        Ok(())
    }
    ...
}