TLS only supports actual restartable ECDH with the legacy code that's going
away, not with the MBEDTLS_USE_PSA_CRYPTO code that's becoming the only
variant. This leaves a few test cases that validate restartable ECDH in TLS
as desirable, but not currently able to pass.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>