[WIP] sdk/log: Fix TestBatchProcessor/ForceFlush/ErrorPartialFlush flaky test

[amanakin]

Fix flaky test issued in #5342

Run of go test -v -timeout 10s -short -count=10000 ./... -run='TestBatchProcessor/ForceFlush/ErrorPartialFlush' Before: Failed with panic: test timed out after 10s After: Passed

Problem was with bufferExporter.input chan. In test expected:

1 export being performed, 1 export in buffer chan, >1 batch

But buffer chan wasn't checked. Now we ensure, that record is in buffer chan