Removed 13 unnecessary stubbings from InstanceConfigurationTest.java

[ARUS2023]

Testing done

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- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
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In our analysis of the project, we observed that

1. 12 unnecessary stubbings which stubbed listRegions method, listZones method, listMachineTypes method, listCpuPlatforms method, listBootDiskTypes method, listImages method, listAcceleratorTypes method, listNetworks method, listSubnetworks method, getProjectId method, getClient method, getTemplate method in init are created but are never executed by 4 tests InstanceConfigurationTest.testInstanceMetadata, InstanceConfigurationTest.testInstanceModel, InstanceConfigurationTest.testConfigRoundtrip, InstanceConfigurationTest.testClient. [See changes in InstanceConfigurationTest.java]

2. 10 unnecessary stubbings which stubbed listRegions method, listZones method, listMachineTypes method, listCpuPlatforms method, listBootDiskTypes method, getImage method, listImages method, listAcceleratorTypes method, listNetworks method, listSubnetworks method in init are created but are never executed by 4 tests InstanceConfigurationTest.descriptorBootDiskSizeValidation, InstanceConfigurationTest.testInstanceModel, InstanceConfigurationTest.testConfigRoundtrip, InstanceConfigurationTest.testClient; [See changes in SecondInstanceConfigurationTest.java]

3. 13 unnecessary stubbings which stubbed listMachineTypes method, listCpuPlatforms method, listBootDiskTypes method, getImage method, listImages method, listAcceleratorTypes method, listNetworks method, listSubnetworks method, getProjectId method, getClient method, getTemplate method in init are created but are never executed by 3 tests InstanceConfigurationTest.descriptorBootDiskSizeValidation, InstanceConfigurationTest.testInstanceMetadata, InstanceConfigurationTest.testClient; [See changes in ThirdInstanceConfigurationTest.java]

4. 11 unnecessary stubbings which stubbed listRegions method, listZones method, listMachineTypes method, listCpuPlatforms method, listBootDiskTypes method, getImage method, listImages method, listAcceleratorTypes method, listNetworks method, listSubnetworks method, getProjectId method, getClient method, getTemplate method in init are created but are never executed by 4 tests InstanceConfigurationTest.testInstanceMetadata, InstanceConfigurationTest.testInstanceModel, InstanceConfigurationTest.testConfigRoundtrip, InstanceConfigurationTest.descriptorBootDiskSizeValidation. [See changes in FourthInstanceConfigurationTest.java]

Unnecessary stubbings are stubbed method calls that were never realized during test execution. Mockito recommends to remove unnecessary stubbings (https://www.javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/exceptions/misusing/UnnecessaryStubbingException.html). We propose below a solution to remove the unnecessary stubbings.