In order to verify signatures given with test certificates in the DigiDoc4 Client, you need to configure the test certificates manually. Otherwise the signature validation check will return the status “Unknown”.
The DigiDoc4 Client uses Trust Service Status Lists (TSLs) to check the trustworthiness of digitally signed BDOC and ASiC-E files. TSLs are used both when signing documents and validating signatures.
BDOC/ASiC-E test signatures can be validated with the digidoc-tool.exe command line program, which is located in the DigiDoc4 Client installation directory. Instructions for using the command line programme for creating and validating test signatures:
https://github.com/open-eid/libdigidocpp/wiki/Using-test-TSL-lists#digidoc-toolexe-utility-program
As an alternative, you can validate BDOC/ASiC-E test signatures with the DigiDoc4 Client if you change the default TSL URL and the TSL signature certificate data in the source code of the underlying Libdigidocpp library used by the application and recompile the application.
Detailed instructions: https://github.com/open-eid/libdigidocpp/wiki/Using-test-TSL-lists#re-compiling-the-library