Fix failing test suites in the `botan-low` package

[jorisdral]

These failing test suites were noticed in #24, and independently in #21

The following are known to fail on Ubuntu with ghc-9.6 and Botan-3.9

• botan-low-cipher-tests: Fix an "insufficient buffer space" bug in Botan.Low.Cipher.cipherUpdate #84
• botan-low-pubkey-decrypt-tests: Fix a bug in public-key encrypt and decrypt functions #79
• botan-low-pubkey-dsa-tests: Fix PubKey.DSA tests #83
• botan-low-pubkey-encrypt-tests: Fix a bug in public-key encrypt and decrypt functions #79
• botan-low-pwdhash-tests: Fix PwdHash tests, review and refactor PwdHash modules #63
• botan-low-srp6-tests: Fix SRP6 tests, review and refactor SRP6 modules #65, related SRP6 functions get broken because allocBytesQuerying calls botan_srp6_server_session_step1 twice #28

The following is known to fail only on MacOS regardless of the GHC version and with Botan-3.9

• botan-low-rng-tests

We should fix these test suites