Cambridge KET tests are valid forever. See here:
and see here (page 2):
which states that for UKVI purposes, under the Test validity column, the Cambridge KET has: "No expiry".
Vantage Siam are also using this as an advantage of the KET test:
- No Expiry Date: KET results are good for life."
Regarding TB tests, the certificate is valid for 6 months from the date of the test. Here's the source:
According to Part 1. Overview:
"If your test shows that you don’t have TB, you’ll be given a certificate which is valid for 6 months. Include this certificate with your UK visa application. "