Explicitly disable sync caching for Synapse (#1259)

All the tests assume that an initial `/sync` will return up-to-date
data.

lib/SyTest/Homeserver/Synapse.pm

@@ -293,6 +293,11 @@ sub start
         # Enable ephemeral message support (MSC2228)
         enable_ephemeral_messages => "true",
 
+        # Disable caching of sync responses to make tests easier.
+        caches => {
+          sync_response_cache_duration => 0,
+        },
+
         $self->{recaptcha_config} ? (
            recaptcha_siteverify_api => $self->{recaptcha_config}->{siteverify_api},
            recaptcha_public_key     => $self->{recaptcha_config}->{public_key},