Fix PrivateDnsDohTest.ExcessDnsRequests not to interfere other tests

Before this change, DnsResolver reconnects the server after
this test ends. This causes other tests being flaky, especially
for those test that check the number of connections. A known
example is running PrivateDnsDohTest.SessionResumption after
PrivateDnsDohTest.ExcessDnsRequests.

Therefore, this change adds some code in the test to prevent it.

Bug: 222023286
Test: run resolv_integration_test64
      --gtest_filter="*SessionResumption*:*ExcessDnsRequests*"
Change-Id: Id11eb8d984128bc2b336d2939ae129eebeb17807