Unnecessary stubbings detected in InstanceConfigurationTest

While running PCT to guarantee the compatibility of the **_google-compute-engine-4.3.0_** with other plugins installed, the CI fails with the error message provided.

It would be great to get rid of this problem because it's breaking `mvn test`

Thank you.

```
Unnecessary stubbings detected in test class: InstanceConfigurationTest
Clean & maintainable test code requires zero unnecessary code.
Following stubbings are unnecessary (click to navigate to relevant line of code):
1. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:160)
2. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:162)
3. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:164)
4. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:167)
5. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:169)
6. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:171)
7. -> at com.google.jenkins.plugins.computeengine.InstanceConfigurationTest.init(InstanceConfigurationTest.java:173)
Please remove unnecessary stubbings or use 'lenient' strictness. More info: javadoc for UnnecessaryStubbingException class.
```