I am aware of a number of people for whom eating foods containing poppy seeds has caused problems. In one case, someone was taken off safety-critical duties for two weeks for further testing until there was solid evidence that the test failure was due to food.
So don't say the tests aren't sensitive. There are some strange but nonetheless probably logical things they pick up - at my last drugs test I had to list any energy drinks (amongst an interesting list of other potentially problematic items) I had consumed within the last week. I usually loathe the things but then I realised I'd had one six days beforehand. However, TOC occupational health will always be able to advise what you can/cannot eat or drink, and what needs to be declared. And if it's anything like any of the medicals/drug tests I've had, you will be countersigning that you're happy with the way the sample(s) were taken anyway, including any substances therein.
Back on topic, I've never heard of any foods including coconut or curry powder to cause problems, but I'm not an occupational health expert and I don't know if they contain passive forms of any substances which can be flagged up as problems. If in doubt, always speak to the person obtaining the samples from you. So long as it's explainable and is not illegal or able to affect you in any way during your duties, you should be able to find a workaround. But in all things the railway nowadays errs on the side of caution, and medical tests likewise.
I believe all tests would be completed in the same amount of time - eg. 48hrs - but there may only be on-call staff in occupational health or their agencies at weekends, so you may find things are flagged up on a Monday morning. Each company would probably vary.