In a PSK-only build:
* Skip tests that rely on a specific non-PSK cipher suite.
* Skip tests that exercise a certificate authentication feature.
* Pass a pre-shared key in tests that don't mind the key exchange type.
This commit only considers PSK-only builds vs builds with certificates. It
does not aim to do something useful for builds with an asymmetric key
exchange and a pre-shared key for authentication.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>