disable TestThreadPoolForkSafety.NestedChild, hangs and gobbles CPU

Index: cpp/src/arrow/util/thread_pool_test.cc
--- cpp/src/arrow/util/thread_pool_test.cc.orig
+++ cpp/src/arrow/util/thread_pool_test.cc
@@ -930,6 +930,9 @@ TEST_F(TestThreadPoolForkSafety, NestedChild) {
 #  ifdef __APPLE__
     GTEST_SKIP() << "Nested fork is not supported on macos";
 #  endif
+#  ifdef __OpenBSD__
+    GTEST_SKIP() << "Test hangs on OpenBSD";
+#  endif
 #  ifndef ARROW_ENABLE_THREADING
     GTEST_SKIP() << "Test requires threading support";
 #  endif
