Test: Deflake ResolverTest#QueryTlsServerTimeout

Occasionally, the test starts the second DNS query while DnsResolver
is still cleaning up the DoT socket created for the first DNS query.
In that case, the second DNS query immediately gets an error from
DoT engine, and then the test fails.

Actually, the second DNS query can be removed from the test, because
the value of dot_query_timeout_ms is verified by the first DNS query.

Therefore, this CL removes the second DNS query. In addition, to
speed up the test, DoT server response time is reduced from 5s to 2s.

Bug: 310556479
Test: run QueryTlsServerTimeout 100 times
Change-Id: I0cea60667f802861bf31fd4ecb0832b278930a93