Deuteronomy 13:4

you shall not hearken to the words of that lying prophet, or him who hath dreamed that dream; for the Lord your God (thereby) trieth you, to know whether you will love the Lord your God with all your heart and with all your soul.

test in that He let him be, and did not kill him. God tests people to reveal the virtue of him who is tested.

FOR THE ETERNAL YOUR G-D TRIETH YOU. This is to state that the purport of the wonder which came to pass and which G-d showed him in his dream or [as the philosophers say] in his kahin, a term referring to the power residing in man to be able to foretell the future. It is so explained by Shmuel ibn Tibbon in his list of “Foreign words.” was G-d’s will in order to test you in your love of Him. Now I have already explained that the term “test” is derived from the point of view of the one who is being tested. It states here to know [for the Eternal your G-d trieth you ‘to know’ whether ye do love … meaning] that it became known in fact what exists in potential and is revealed [only] to Him. Thus the commandment of this entire section refers to someone who prophesies to worship the idols. But concerning one who prophesies in the name of G-d to change any of the commandments of the Torah, our Rabbis have interpreted that if he came to [completely] uproot any item from the Torah, such as permitting [the eating of] swine or [permitting] one of the adulterous relationships, according to this judgment shall it be done to him [i.e., he is liable to death as is stated here in Verse 6]. But if he declares it to be permissible only for some time as a temporary measure such as Elijah did on Mount Carmel [in permitting offerings to G-d outside of the Sanctuary in Jerusalem], 9:3). we are obligated to listen to him and to do everything that he commands us.
It appears [to me] that if he prophesies in the name of G-d to command a precept that he innovates, as happened concerning reading of the Scroll [of Esther on Purim], that he is not guilty of a capital offense, only that we are not to obey him, because it is written, These are the commandments, [which our Rabbis interpret to mean]:. The reading of the Megillah (Scroll of Esther) on Purim is “a positive commandment by the authority of the Scribes” (Mishneh Torah, Hilchoth Megillah V’Chanukah 1:1). “No prophet is henceforth permitted to declare any new matter therein.” But perhaps, since we do not believe him, he is thus a false prophet and his death is by strangulation.

'כי מנסה ה' וגו, the only reason G’d allowed such people to possess what sounds like supernatural knowledge, is to test our faith, to see if we will heed the prohibition against using such sources of information and of tolerating such people as part of our culture. By choosing not to resort to the knowledge of such people of the future, etc., we accumulate merits in G’d’s books (compare Deut. 18,10-13). Refusing to believe such false prophets is in itself a good deed. Believing them is equivalent to believing in idolatry.

לא תשמע אל הנביא ההוא, do not even turn to listen to find out if his words have even partial substance, for there can be no question that the man is full of lies and tells you things he has made up, all for an evil purpose; כי דבר סרה, for what he portrays as a miracle is nothing but sleight of hand or forgery of some other kind.

או אל חולם החלום ההוא, to see if even part of what he dreamt reflected some truth of what was going to happen; you may be sure that the dreamers did not actually dream any part of what he claimed to have dreamt, but he invented all of it in order to seduce you;

כי מנסה ה' אלוקיכם אתכם, by referring with hostility to a person who spoke out against G’d, you will be proven as truly loving G’d.

לדעת, once your previously theoretical faith in G’d has been so tested, you will become truly tested in your intimate conviction of G’d’s goodness which becomes an unshakable enduring conviction. Someone who believes truly in the goodness of the Lord cannot help but hate anyone who denies G’d’s goodness, just as if you love a human being truly you cannot help hating a person who hates the person whom you truly love.