The testimony of Jesus as used in D&C 76 refers to a testimony of Christ as he really is. It is not about the many Christian churches that stand in opposition to the true church. Of these various denominations Christ actually declares:
D&C 76:98 And the glory of the telestial is one, even as the glory of the stars is one; for as one star differs from another star in glory, even so differs one from another in glory in the telestial world;
99 For these are they who are of Paul, and of Apollos, and of Cephas.
100 These are they who say they are some of one and some of another—some of Christ and some of John, and some of Moses, and some of Elias, and some of Esaias, and some of Isaiah, and some of Enoch;
101 But received not the gospel, neither the testimony of Jesus, neither the prophets, neither the everlasting covenant.
For all practical purposes receiving the testimony of Jesus in D&C 76 can be replaced with stating that one becomes a member of the church of Jesus Christ of Latter-day Saints.
Thus we read that those who were celestial
D&C 76:50 And again we bear record—for we saw and heard, and this is the testimony of the gospel of Christ concerning them who shall come forth in the resurrection of the just—
51 They are they who received the testimony of Jesus, and believed on his name and were baptized after the manner of his burial, being buried in the water in his name, and this according to the commandment which he has given—
But that among the Terrestrial we find:
D&C 76:72 Behold, these are they who died without law;
73 And also they who are the spirits of men kept in prison, whom the Son visited, and preached the gospel unto them, that they might be judged according to men in the flesh;
74 Who received not the testimony of Jesus in the flesh, but afterwards received it.
75 These are they who are honorable men of the earth, who were blinded by the craftiness of men.
And we have already recalled the relevant references to the telestial.