RFR: 8368694: PKCS11-NSS generic keys generated by DH have leading zeroes stripped

[Daniel Jeliński]

The DiffieHellman KeyAgreement supports 2 key algorithms: TlsPremasterSecret and Generic. The Generic algorithm is supposed to generate keys of a constant length, keeping leading zeroes as appropriate.

This PR changes the SunPKCS11 implementation to pass a CKA_VALUE_LEN attribute when a fixed length is needed; when the attribute is absent, the PKCS11 provider strips the leading zeroes.

Added a check to the existing test cases to verify the fix. The check passes with the fix, fails without it. Other tier1-3 tests continue to pass.

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Commit messages:
 - Fix leading zero handling
 - Add tests for leading zero handling

Changes: https://git.openjdk.org/jdk/pull/27494/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27494&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368694
  Stats: 71 lines in 3 files changed: 50 ins; 13 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/27494.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27494/head:pull/27494

PR: https://git.openjdk.org/jdk/pull/27494