MBEDTLS_PK_SIGNATURE_MAX_SIZE is tested in Mbed Crypto. Its effect on
Mbed TLS is also tested via the X.509 tests. The case of
MBEDTLS_MPI_MAX_SIZE < MBEDTLS_ECDSA_MAX_LEN, for which this component
was added as a regression test, is covered by config-suite-b.h which
is tested via test-ref-configs.pl.