Fix tests on macOS with Homebrew libmagic

• Remove ei_init() call that fails when sysconf(_SC_OPEN_MAX) returns LONG_MAX (unlimited fd limit on macOS), causing int overflow in Erlang's init_socket_info(). The port only uses ei for encoding/decoding, not node connections.
• Redirect port stderr to /dev/null unless MAJIC_DEBUG is set, to suppress libmagic warnings in test output.
• Compile test .mgc with matching file binary from Homebrew file-formula to avoid version mismatch with the linked libmagic.
• Increase port test receive timeout to accommodate slower port startup.
• Broaden .gitignore to cover /priv/ and *.mgc.