tests/qtest/migration-test: Disable migration/multifd/tcp/plain/cancel

migration-test has been flaky for a long time, both in CI and
otherwise:

https://gitlab.com/qemu-project/qemu/-/jobs/3806090216
(a FreeBSD job)
  32/648 ERROR:../tests/qtest/migration-helpers.c:205:wait_for_migration_status: assertion failed: (g_test_timer_elapsed() < MIGRATION_STATUS_WAIT_TIMEOUT) ERROR

on a local macos x86 box:
▶  34/621 ERROR:../../tests/qtest/migration-helpers.c:151:migrate_query_not_failed: assertion failed: (!g_str_equal(status, "failed")) ERROR
 34/621 qemu:qtest+qtest-i386 / qtest-i386/migration-test                         ERROR          168.12s   killed by signal 6 SIGABRT
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stderr:
qemu-system-i386: Failed to peek at channel
query-migrate shows failed migration: Unable to write to socket: Broken pipe
**
ERROR:../../tests/qtest/migration-helpers.c:151:migrate_query_not_failed: assertion failed: (!g_str_equal(status, "failed"))

(test program exited with status code -6)
――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――

▶  37/621 ERROR:../../tests/qtest/migration-helpers.c:151:migrate_query_not_failed: assertion failed: (!g_str_equal(status, "failed")) ERROR
 37/621 qemu:qtest+qtest-x86_64 / qtest-x86_64/migration-test                     ERROR          174.37s   killed by signal 6 SIGABRT
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stderr:
query-migrate shows failed migration: Unable to write to socket: Broken pipe
**
ERROR:../../tests/qtest/migration-helpers.c:151:migrate_query_not_failed: assertion failed: (!g_str_equal(status, "failed"))

(test program exited with status code -6)

In the cases where I've looked at the underlying log, this seems to
be in the migration/multifd/tcp/plain/cancel subtest.  Disable that
specific subtest by default until somebody can track down the
underlying cause. Enthusiasts can opt back in by setting
QEMU_TEST_FLAKY_TESTS=1 in their environment.

We might need to disable more parts of this test if this isn't
sufficient to fix the flakiness.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20230302172211.4146376-1-peter.maydell@linaro.org

tests/qtest/migration-test.c

@@ -2572,8 +2572,14 @@ int main(int argc, char **argv)
     qtest_add_func("/migration/auto_converge", test_migrate_auto_converge);
     qtest_add_func("/migration/multifd/tcp/plain/none",
                    test_multifd_tcp_none);
-    qtest_add_func("/migration/multifd/tcp/plain/cancel",
-                   test_multifd_tcp_cancel);
+    /*
+     * This test is flaky and sometimes fails in CI and otherwise:
+     * don't run unless user opts in via environment variable.
+     */
+    if (getenv("QEMU_TEST_FLAKY_TESTS")) {
+        qtest_add_func("/migration/multifd/tcp/plain/cancel",
+                       test_multifd_tcp_cancel);
+    }
     qtest_add_func("/migration/multifd/tcp/plain/zlib",
                    test_multifd_tcp_zlib);
 #ifdef CONFIG_ZSTD