sure people can manipulate tests but when you think about it if they get diagnosed with something they dont really have the treatment for the misdiagnosis wont work.

example

someone wants to be adhd lies on question forms. gets ritalin, concerta and other adhd meds they are going to be going as if someone dropped them some acid, speed. the treatment for adhd is giving stimulants, uppers type medications.

someone wants to be DID the doctors tell them heres some meds that should work knowing full well there is no medication that works for DID and gives the person an antipsychotic that takes away schizophrenia symptoms. the next appointment the person reports yes the meds are working the doctor knows the person wasnt DID.

doctors all the time do things like giving placebos telling the person the meds will make their problems better just to see if they were manipulating and lying. if the problem is solved with a "sugar pill" then the doctors know the person wasnt being truthful on the tests.

in my psych classes I am learning alot of different ways doctors can tell when someone hasnt been truthful on tests. Like Doc John said one of those ways is the tests themselves have built in questions that reviel whether someone is trying to manipulate the testing.