Add more test for tx observers #455

Job	Run time
dci-lint / DCI-Lint	23s
unit-tests / Unit Tests (18.x)	40s
unit-tests / Unit Tests (20.x)	30s
coverage / Coverage Reports (caliper-core)	6s
integration-tests / Check for changes	2s
coverage / Coverage Reports (caliper-fabric)	5s
coverage / Coverage Reports (generator-caliper)	8s
integration-tests / Integration Tests (fabric, 18.x)	6h 0m 15s
integration-tests / Integration Tests (fabric, 20.x)	6h 0m 15s
integration-tests / Integration Tests (ethereum, 18.x)	1m 37s
integration-tests / Integration Tests (ethereum, 20.x)	1m 37s
integration-tests / Integration Tests (besu, 18.x)	2m 50s
integration-tests / Integration Tests (besu, 20.x)	2m 45s
integration-tests / Integration Tests (generator, 18.x)	3m 5s
integration-tests / Integration Tests (generator, 20.x)	2m 50s
	12h 17m 8s

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