[fit][utest] Disable abandoning_tasks test

Disable flaky test test abandoning_tasks to unblock CQ.

Bug: FLK-260 #comment disable test abandoning_tasks
Change-Id: Ia1910f29cdb0f135b47d0d57a4576b05c48afd24
diff --git a/zircon/system/utest/fit/single_threaded_executor_tests.cpp b/zircon/system/utest/fit/single_threaded_executor_tests.cpp
index f3861d0..908828a 100644
--- a/zircon/system/utest/fit/single_threaded_executor_tests.cpp
+++ b/zircon/system/utest/fit/single_threaded_executor_tests.cpp
@@ -227,7 +227,8 @@
 BEGIN_TEST_CASE(single_threaded_executor_tests)
 RUN_TEST(running_tasks)
 RUN_TEST(suspending_and_resuming_tasks)
-RUN_TEST(abandoning_tasks)
+// Test disabled due to flakiness.  See FLK-260.
+// RUN_TEST(abandoning_tasks)
 RUN_TEST(run_single_threaded)
 RUN_TEST(run_single_threaded_move_only_result)
 END_TEST_CASE(single_threaded_executor_tests)