Fix order dependent tests in SofaRpcFallbackRegistryTest

[SaaiVenkat]

Describe what this PR does / why we need it

• In sentinel-adapter/sentinel-sofa-rpc-adapter, the unit test SofaRpcFallbackRegistryTest.testDefaultFallback() will fail when run after the unit test SofaRpcFallbackRegistryTest.testCustomFallback() because it pollutes state shared among tests.
• It may be good to clean this state pollution so that this tests and some other tests do not fail in the future due to the shared state polluted by this test.

Does this pull request fix one issue?

Fixes #3281

Describe how you did it

• As mentioned in the issue, I have cleaned the polluted states SofaRpcFallback.providerFallback and SofaRpcFallback.consumerFallback to its default value after the test SofaRpcFallbackRegistryTest.testCustomFallback() is executed.

Describe how to verify it

• With the proposed fix, the test does not pollute the shared state.
• All the tests pass when run in the any order.

[LearningGp]

LGTM

[LearningGp]

Thanks for contributing!