Thank you, my own view – bear in mind that I have never worked in a commercial laboratory – is that the scientists doing the testing are likely to be honest in their work. However, they can only work with what they are given, so it could be that the samples were tampered with before being handed to the labs for testing.
With regard to tests for chemicals. There are very well established methods. There is really nothing that can escape the net in the sense that you cannot make something invisible to tests. If a particular chemical residue is present then it can be detected. So there would be no sense in tampering with lab samples to test the capabilities of a lab. They all use the same methods, they should all produce the same results.
I am going to write something on the Skripals on the Sturgess Inquest thread.