No, it isn't. The oath reads,
"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
To preserve, protect and defend the Constitution requires enforcing the laws of the country that are passed under that Constitution. Trump enforces those laws, even if Democrats don't like it, with our immigration laws being a major focus today.
The single arbiter of what's legal under our Constitution, is the Supreme Court. Trump does what he believes is necessary to enforce those laws. Others react in one of two ways. They either rant, piss and moan because he doesn't cave in to what they want him to do, or they sue. Suing brings the dispute where it belongs so that the Constitutionality of the action or the law is confirmed or, if not confirmed, serves as a guide to the Legislative and Administrative branches to revise the law.
IOW, testing a law or an action doesn't violate the oath. By clarifying the law, it strengthens the Constitution.