Add more test for tx observers #458

Job	Run time
dci-lint / DCI-Lint	27s
unit-tests / Unit Tests (18.x)	43s
unit-tests / Unit Tests (20.x)	35s
integration-tests / Check for changes	2s
coverage / Coverage Reports (caliper-core)	6s
coverage / Coverage Reports (caliper-fabric)	8s
coverage / Coverage Reports (generator-caliper)	5s
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 33s
integration-tests / Integration Tests (ethereum, 20.x)	1m 33s
integration-tests / Integration Tests (besu, 18.x)	2m 43s
integration-tests / Integration Tests (besu, 20.x)	2m 48s
integration-tests / Integration Tests (generator, 18.x)	2m 35s
integration-tests / Integration Tests (generator, 20.x)	2m 48s
	12h 16m 36s

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