Make docker packaging test more resilient (elastic#111205)

Wrap check for container shutdown log message in an `assertBusy()` to
deal with race conditions.

Closes elastic#111132
# Conflicts:
#	muted-tests.yml

qa/packaging/src/test/java/org/elasticsearch/packaging/test/DockerTests.java

@@ -1231,7 +1231,7 @@ public void test500Readiness() throws Exception {
         assertBusy(() -> assertTrue(readinessProbe(9399)));
     }
 
-    public void test600Interrupt() {
+    public void test600Interrupt() throws Exception {
         waitForElasticsearch(installation, "elastic", PASSWORD);
         final Result containerLogs = getContainerLogs();
 
@@ -1241,10 +1241,12 @@ public void test600Interrupt() {
         final int maxPid = infos.stream().map(i -> i.pid()).max(Integer::compareTo).get();
 
         sh.run("bash -c 'kill -int " + maxPid + "'"); // send ctrl+c to all java processes
-        final Result containerLogsAfter = getContainerLogs();
 
-        assertThat("Container logs should contain stopping ...", containerLogsAfter.stdout(), containsString("stopping ..."));
-        assertThat("No errors stdout", containerLogsAfter.stdout(), not(containsString("java.security.AccessControlException:")));
-        assertThat("No errors stderr", containerLogsAfter.stderr(), not(containsString("java.security.AccessControlException:")));
+        assertBusy(() -> {
+            final Result containerLogsAfter = getContainerLogs();
+            assertThat("Container logs should contain stopping ...", containerLogsAfter.stdout(), containsString("stopping ..."));
+            assertThat("No errors stdout", containerLogsAfter.stdout(), not(containsString("java.security.AccessControlException:")));
+            assertThat("No errors stderr", containerLogsAfter.stderr(), not(containsString("java.security.AccessControlException:")));
+        });
     }
 }