Addresses https://github.com/Mbed-TLS/mbedtls/issues/9586 .
This is not a fully satisfactory resolution, because we don't run every
constant-flow test with Valgrind in PR jobs, only a small subset. We should
improve the coverage/resource balance.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>