Add more tests for caliper worker #492

Job	Run time
dci-lint / DCI-Lint	28s
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)	8s
coverage / Coverage Reports (caliper-fabric)	4s
coverage / Coverage Reports (generator-caliper)	8s
integration-tests / Integration Tests (fabric, 18.x)	8m 27s
integration-tests / Integration Tests (fabric, 20.x)	8m 22s
integration-tests / Integration Tests (ethereum, 18.x)	2m 14s
integration-tests / Integration Tests (ethereum, 20.x)	2m 13s
integration-tests / Integration Tests (besu, 18.x)	4m 22s
integration-tests / Integration Tests (besu, 20.x)	4m 22s
integration-tests / Integration Tests (generator, 18.x)	2m 56s
integration-tests / Integration Tests (generator, 20.x)	2m 38s
	37m 31s

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