xdsclient/test: use separate channels in test watchers for error, update and resource not exist

[purnesh42H]

The current end-to-end tests for xdsclient utilize a watcher implementation with a channel size of 1. This limitation prevents us from testing scenarios involving multiple updates with a combination of errors and successful updates.

To overcome this, we should modify the test watcher implementations to incorporate three separate channels:

• Update Channel: For successful updates
• Error Channel: For error updates
• Resource Not Found Channel: For "resource not found" updates

Each channel should have a configurable size, allowing tests to adjust it as needed. This enhancement will enable comprehensive testing of various update scenarios, including those with mixed successes and failures.