Set MBEDTLS_TEST_PLATFORM and MBEDTLS_TEST_CONFIGURATION to meaningful
values in all.sh. These environment variables are used when
writing an outcome file, which happens if MBEDTLS_TEST_OUTCOME_FILE is
also set.
When running one of the try-multiple-configuration scripts, set
MBEDTLS_TEST_CONFIGURATION to a value that uniquely describes the
configuration.
This is step 1 of a plan to get rid once and for all of missing depends_on in
the X509 test suite (step 2 will be RSA/ECDSA, and step 0 was curves.pl).