He would just not show up.
see #7
It doesn’t have to work that way the house chose not to enforce some of those subpoenas in court, trump would ask the chief Justice on the spot to enforce the subpoena which a trial judge has an absolute right to do, or force trump to file that request with a district court. No matter what, a court would decide the issue and order Biden testify.
In the middle of the trial I would see no reason for the issue to go anywhere but to the presiding judge which would be John Roberts.