fix flaky test CloseRelpFrameServerRXConsumerTest.testSendMessage

[kortemik]

Description
CI reports

Error:    CloseRelpFrameServerRXConsumerTest.testSendMessage:130 expected: <true> but was: <false>

this is likely to be due to async closing of the FrameProcessorPool

[StrongestNumber9]

I wasn't able to reproduce it locally without edits but it is 100% trigger rate with this edit

--- a/src/test/java/com/teragrep/rlp_03/CloseRelpFrameServerRXConsumerTest.java
+++ b/src/test/java/com/teragrep/rlp_03/CloseRelpFrameServerRXConsumerTest.java
@@ -80,6 +80,7 @@ public class CloseRelpFrameServerRXConsumerTest {
 
         @Override
         public void close() throws Exception {
+            Thread.sleep(1000);
             closed.set(true);
         }
     }