Add wire subject test coverage

[psiman-wandera]

Adding test coverage for new Wire Subject related methods. This improves the overall test coverage of critical parts of the order.Finalize method

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[psiman-wandera]

Hi @hslatman, @beltram , @stefanwire - would you be so kind and have a look at this test coverage of methods related to Wire Subject you were working on lately? Thanks

[psiman-wandera]

@1nkfreax can you please re-approval. I've just rebased the branch. Sorry for long delay.

[hslatman]

Test failure is unrelated. Fixing in #2235.

[hslatman]

Thank you, @psiman-wandera! 😄