What is the test for a warranty?

See also Kerr on Fraud and Mistake, 4th ed., at p. 38: In order that a statement or representation may amount to a warranty, it must appear that it was intended to form a substantive part of the contract. (Behn v. Burness, 3 B. & S. 751, at p. 753, 32 L.J.Q.B. 204.) A warranty is an express or implied statement of something which the party making it undertakes shall be a substantive part of the contract, and though part of the contract, yet collateral to the express object of it.

Also at p. 39: Any affirmance or representations made at the time of sale is a warranty if it appears to have been so intended and understood by the parties; and in determining whether it was so intended a decisive test is whether the vendor assumes to assert a fact of which the buyer is ignorant, or merely states an opinion on a matter of which the vendor has no special knowledge, and on which the buyer may be expected to exercise his own judgment. (De Lassalle v. Guildford [1901] 2 K.B. 215, 70 L.J.K.B. 533.) Nor is it necessary that the statement or representation should be simultaneous with the close of the bargain;

At p. 40: If it be part of the contract, it matters not at what period of the negotiations it was made. (Hopkins v. Tanqueray, 15 C.B. 130, at p. 137, 2 C.L.R. 842, 23 L.J.C.P. 162.) If a statement amounts to a warranty the party making it is bound by his warranty. The fact that he may have made the statement in honest mistake, or that the statement may be not in a material matter, cannot be taken into consideration.