Increase TEST_TIMEOUT_MS used in the integration test.

Increase the timeout from 400ms to 2s to preclude the flaky failure
due to the callback to be verified will hasn't been invoked before the
test case ends. For example, mCb.onQuit is wanted but not invoked in
some flaky tests, investigating the bugreport, the transition to
StoppingState takes 400ms+ sometimes after introducing the metrics
record implementation, then test just ends before execution of the
callback. Besides, increasing timeout to 2s doesn't bring extra time
consumption, the total consumed time comparison as below:

before:
time atest -ti NetworkStackIntegrationTests:IpClientIntegrationTest
real	0m44.292s

after:
time atest -ti NetworkStackIntegrationTests:IpClientIntegrationTest
real	0m44.802s

Bug: 163913194
Test: atest NetworkStackIntegrationTests
Change-Id: I093673cb1989a7e18545ef48b9bcc46b56fbbbfd