Expand negative coverage of ECDSA verification
Expand negative coverage of ECDSA verification

Motivated by CVE-2022-21449, to which we're not vulnerable, but we
didn't have a test for it. Now we do.

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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