Even after commit 1bbc74b55 ("REGTEST: fix tls_health_checks random
failures on MacOS in Travis-CI") which extended it to 100ms, it still
randomly fails on Travis, so let's push it to 500ms and mark it slow.
Since commit 2eb1c79df ("REGTEST: make the tls_health_checks test much
faster") the build tests randomly fail on MacOS on Travis-CI. Each time
this test is reponsible for the failure, showing huge response times
possibly indicating that the VMs running the tests are sometimes
overloaded. Since this delay directly impacts the whole regtest execution
time everywhere, it's important not to inflate it too much. It was bumped
to 100ms instead of 40, that doesn't add significantly to the perceived
execution time and should be enough for Travis since test reports have
shown around 60-70 ms.
This test relies on a server timeout and was using the default 2s check
interval with a full 1s server timeout, thus adding a whole second to the
test series by itself. Let's shrink the server timeout to 20ms which is
way enough to properly trigger a timeout, and set the check interval to
the double of this, or 40ms.