The test environment doesn't have OpenResty libraries, so we need
to provide mock implementations for testing.
Created:
- tests/mock_resty_sha256.lua: Uses system sha256sum command to
compute SHA-256 hashes. Mimics the resty.sha256 API (new,
update, final).
- tests/mock_resty_string.lua: Implements to_hex() to convert
binary strings to hexadecimal.
Updated test.lua to preload these mocks so that when the module
or tests require 'resty.sha256' or 'resty.string', they get our
mock implementations instead.
This allows the PoW verification tests to run and actually verify
the SHA-256 proof-of-work.
