Pad the RandomSeed to avoid CredScan false positive

[heaths]

For tests that use Recording::random(), the RandomSeed is stored as a base64-encoded 256 bit value that gets detected as part of a SAS token. Suppressing this across various assets is more problematic, so it's better we instead just pad the RandomSeed bytes that we store and trim them off before use e.g., we could add br"test" before encoding and, if the value we read is more than 256 bits we just trim it off. This should be backward compatible so we don't have to re-recorded, but will fix itself in time when we do incidentally re-record.