Add a build with MBEDTLS_ERROR_STRERROR_DUMMY but not MBEDTLS_ERROR_C.
Previously, both options were enabled by default, but
MBEDTLS_ERROR_STRERROR_DUMMY only matters when MBEDTLS_ERROR_C is
enabled, so its effect was not tested.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
d754c8bad6