Speed up the generation of storage format test cases
Speed up the generation of storage format test cases

Restore the optimization done in
 HEAD^{/Speed up the generation of storage format test cases}
which was lost during refactoring made when adding support for
implicit usage flags.

There are still more than one call to the C compiler, but the extra
calls are only for some key usage test cases.

This is an internal refactoring. This commit does not change the
output of generate_psa_tests.py

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls