These are ones likely to fail on heavily-loaded machines that alter the
normal test timing. Most of these tests already had the flaky keyword
since this condition makes them more likely to fail on CI.
Added Curl_resolver_kill() for all three resolver modes, which only
blocks when necessary, along with test 1592 to confirm
curl_multi_remove_handle() doesn't block unless it must.
Closes#3428Fixes#3371
