selftest: Skip tests known to be flaky on z/OS

Throughput tests for non-blocking AF_UNIX datagram channels seem to not
always work as expected on z/OS.

Skip these tests on z/OS for the time being.

junixsocket-common/src/test/java/org/newsclub/net/unix/domain/ThroughputTest.java

@@ -17,6 +17,9 @@
  */
 package org.newsclub.net.unix.domain;
 
+import static org.junit.jupiter.api.Assumptions.assumeFalse;
+
+import org.junit.jupiter.api.Test;
 import org.newsclub.net.unix.AFSocketCapability;
 import org.newsclub.net.unix.AFSocketCapabilityRequirement;
 
@@ -34,4 +37,20 @@ public ThroughputTest() {
   protected String stbTestType() {
     return "AF_UNIX";
   }
+
+  @Override
+  @Test
+  public void testDatagramChannelNonBlocking() throws Exception {
+    assumeFalse("z/OS".equals(System.getProperty("os.name", "")),
+        "KNOWN ISSUE: Test may fail on z/OS");
+    super.testDatagramChannelNonBlocking();
+  }
+
+  @Override
+  @Test
+  public void testDatagramChannelNonBlockingDirect() throws Exception {
+    assumeFalse("z/OS".equals(System.getProperty("os.name", "")),
+        "KNOWN ISSUE: Test may fail on z/OS");
+    super.testDatagramChannelNonBlockingDirect();
+  }
 }