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>

tests/qtest/migration-test.c

@@ -2912,14 +2912,8 @@ int main(int argc, char **argv)
     }
     qtest_add_func("/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")) {
     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