Fix order dependent test in handler test

[SaaiVenkat]

Issue

• In com.networknt.handler.HandlerTest, the unit test validConfig_init_handlersCreated() can trigger java.lang.RuntimeException when being run before the unit test invalidMethod_init_throws() in the same class because it pollutes state shared among tests.
• It may be good to clean this state pollution so that some other tests do not fail in the future due to the shared state polluted by this test.

Fixes #2005

Solution

• Resetting the HandlerConfig to default value before each test run using the Junit's @Before annotation, where the configuration is set to default value through Handler.setConfig('handler')
• Adding initChain() because initPath() in setConfig, which expects handler or chain to already be created.

[SaaiVenkat]

Greetings @stevehu, similar to #856, this PR also fixes the order dependency issue. I am happy to hear your feedback and discuss more about this issue.

[stevehu]

@SaaiVenkat It makes sense. Thanks a lot for your help.