Add passthrough for pure ML-DSA to the test driver

We don't support ML-DSA in libtestdriver1 yet, because it's a copy of the
`builtin` driver but ML-DSA is provided by the `pqcp` driver. This means
that we can't test “driver-only” ML-DSA builds, but it should be possible to
enable ML-DSA in a build that dispatches through the test driver. This is
currently impossible because pure ML-DSA is not a sign-the-hash algorithm,
but the code in the test driver for signatures assumes that all signature
algorithms are sign-the-hash. Fix this in a minimal way by making the test
driver activate the fallback mechanism of driver dispatch when the algorithm
is pure ML-DSA. (Don't do this for all algorithms that are not sign-the-hash,
because in general, we do want the test driver to fail if it's given an
algorithm that it doesn't support.)

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>