REGTEST: extend the check duration on tls_health_checks and mark it slow

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.
diff --git a/reg-tests/checks/tls_health_checks.vtc b/reg-tests/checks/tls_health_checks.vtc
index 3c0aef2..4a431af 100644
--- a/reg-tests/checks/tls_health_checks.vtc
+++ b/reg-tests/checks/tls_health_checks.vtc
@@ -1,5 +1,6 @@
 varnishtest "Health-check test over TLS/SSL"
 #REQUIRE_OPTIONS=OPENSSL
+#REGTEST_TYPE=slow
 feature ignore_unknown_macro
 
 
@@ -79,7 +80,7 @@
         timeout client 20
         timeout server 20
         timeout connect 20
-        default-server downinter 1s inter 100 rise 1 fall 1
+        default-server downinter 1s inter 500 rise 1 fall 1
 
     backend be2
         option log-health-checks