Increase TEST_TIMEOUT_MS to 30s in the IpClientTest.

Generally TEST_TIMEOUT_MS constant is used to wait particular event
to happen or verify API to be invoked. So far this constant is 400ms,
probably that is not long enough on certain test platform. updating
it to 30s in unit test doesn't increase the entire test duration.

Bug: 202548741
Test: atest NetworkStackTests NetworkStackNextTests
Change-Id: I89f8f4752f9edc6246ea8d998ef26459b25801a9