In response to feedback[1], we can reuse much of the functions in
similar test cases by specifying some additional parameters.
Specifically, test cases which probe the functionality of
`mbedtls_pkcs7_parse_der` have all been merged into one test function.
Additionally, all test cases which examine the
`mbedtls_pkcs7_signed_data_verify` and `mbedtls_pkcs7_signed_hash_verify`
functions have been merged into two test functions (one for single and one
for multiple signers).
[1] https://github.com/Mbed-TLS/mbedtls/pull/3431#discussion_r953686780
Signed-off-by: Nick Child <nick.child@ibm.com>
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