Fix #430: Prevent "Connection unexpectedly closed" in reactive tests

This is an attempt to fix it, according to
spring-projects/spring-data-r2dbc#810.

Because it's not directly possible to reproduce the issue, we can try it
and see if the issue occurs again.

outbox-kafka-spring-reactive/src/test/java/one/tomorrow/transactionaloutbox/reactive/AbstractIntegrationTest.java

@@ -54,6 +54,8 @@ public static void setR2DBCProperties(DynamicPropertyRegistry registry) {
         registry.add("spring.r2dbc.pool.enabled", () -> "true");
         registry.add("spring.r2dbc.pool.initial-size", () -> "5");
         registry.add("spring.r2dbc.pool.max-size", () -> "10");
+        // disable background disposal of idle connections, to prevent "Connection unexpectedly closed"
+        registry.add("spring.r2dbc.pool.max-idle-time", () -> "-1");
     }
 
 }