This is a complicated question and the answer would really depend upon what the Employer's Works Information said; what the Contractor's Works Information said as well as whether option X15 was used.
However, in principle, I believe it could become a latent defect but the Employer would lose the obvious ability to reclaim - by not paying back - the the costs of getting it corrected.
A compromise - ideally by agreement, but you could say it is a Defect already - could be to make an estimate of the most likely cost to repair if the test fails and deduct that amount you pay back from the Contractor. If it passes, then pay that money back.
However, in principle, I believe it could become a latent defect but the Employer would lose the obvious ability to reclaim - by not paying back - the the costs of getting it corrected.
A compromise - ideally by agreement, but you could say it is a Defect already - could be to make an estimate of the most likely cost to repair if the test fails and deduct that amount you pay back from the Contractor. If it passes, then pay that money back.