Add more tests for caliper worker #478

Job	Run time
dci-lint / DCI-Lint	24s
unit-tests / Unit Tests (18.x)	36s
unit-tests / Unit Tests (20.x)	31s
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)	8m 30s
integration-tests / Integration Tests (fabric, 20.x)	8m 37s
integration-tests / Integration Tests (ethereum, 18.x)	2m 24s
integration-tests / Integration Tests (ethereum, 20.x)	2m 22s
integration-tests / Integration Tests (besu, 18.x)	4m 23s
integration-tests / Integration Tests (besu, 20.x)	4m 20s
integration-tests / Integration Tests (generator, 18.x)	2m 52s
integration-tests / Integration Tests (generator, 20.x)	2m 53s
	38m 13s

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