Use mocked Apollo Client instead of $apollo in agents_spec.js

What does this MR do and why?

Use mocked Apollo Client in spec/frontend/clusters_list/components/agents_spec.js

Related to &7436

Convert $apollo mocks in tests to using mocked Apollo Client

Our recommended strategy for testing Vue components with Apollo operations now is mocking Apollo Client in tests. However, there are a few specs where we still mock $apollo with mocks property when mounting a component. These tests should be refactored to using mocked Apollo client. This would also allow us to remove the unnecessary createLocalVue calls from tests

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Related to #384585