Make the signertest work better with the portable test framework

Adjust the signer test to link statically and work with hidden tls_signer
API.

Use TLS_PADDING_* defines.

Revise/simplify for signer interface change.

Add test coverage for tls_signer when used with a TLS server.

In this configuration the tls_signer is provided with the server
certificate and private key, while the TLS server is configured with a

Add test coverage for tls_signer when used with a TLS server.

In this configuration the tls_signer is provided with the server
certificate and private key, while the TLS server is configured with a
sign callback and is only provided with the certificate.

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Add initial regress for tls_signer.