Sleep 3 sec to wait to start HTTP server

[YaSuenag]

GHA reported following error:

[INFO] Running com.yasuenag.checkpointer.test.CheckpointerAgentTest
Error:  Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.530 s <<< FAILURE! -- in com.yasuenag.checkpointer.test.CheckpointerAgentTest
Error:  com.yasuenag.checkpointer.test.CheckpointerAgentTest.testCheckpointerAgent -- Time elapsed: 0.527 s <<< ERROR!
java.io.IOException: HTTP/1.1 header parser received no bytes
	at java.net.http/jdk.internal.net.http.HttpClientImpl.send(HttpClientImpl.java:570)
	at java.net.http/jdk.internal.net.http.HttpClientFacade.send(HttpClientFacade.java:119)
	at com.yasuenag.checkpointer.test.CheckpointerAgentTest.testCheckpointerAgent(CheckpointerAgentTest.java:104)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)