This is the first attempt to test of the constitutionality of Mueller’s appointment.
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Maybe. I do wonder about the ultimate goal of asking a judge to find your client in contempt.
Maybe this was the only way to knock the case up to the Appellate level. If he was told to testify or else, what do you do?
It's the only way to get appellate review.
Here's how the process works: A prosecutor calls someone to testify before the grand jury. The witness moves to quash the subpoena on some ground (here, it's the constitutionality of Mueller's appointment; more commonly it's some issue like attorney-client privilege). If the judge denies the motion, that denial is not an appealable order. The only way to get appellate review is for the witness to refuse to comply and be held in contempt. A contempt finding is appealable.
There must be an adverse ruling to be appealed.