tests/qtest: Re-enable multifd cancel test

We've found the source of flakiness in this test, so re-enable it.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20230606144551.24367-4-farosas@suse.de
[peterx: rebase to 2a61a6964c, to use migration_test_add()]
Signed-off-by: Peter Xu <peterx@redhat.com>

tests/qtest/migration-test.c

@@ -3550,14 +3550,8 @@ int main(int argc, char **argv)
     }
     migration_test_add("/migration/multifd/tcp/plain/none",
                        test_multifd_tcp_none);
-    /*
-     * 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")) {
-        migration_test_add("/migration/multifd/tcp/plain/cancel",
-                           test_multifd_tcp_cancel);
-    }
+    migration_test_add("/migration/multifd/tcp/plain/cancel",
+                       test_multifd_tcp_cancel);
     migration_test_add("/migration/multifd/tcp/plain/zlib",
                        test_multifd_tcp_zlib);
 #ifdef CONFIG_ZSTD