1. Are these tests safe?
The point of these benchmarks is to stress the component to its max to insure stability and that it will not fail. If your system fails on a stress test, you know somehting is wrong.
2. Or it will damage GPU by overvoltage?
Note sure what you mean. In a stress test, as said before, will run it at its max performance. The manufacter specified voltage set will be the voltage it runs on. Overvolting will only really occur if you are trying to OC your component and need more power.
3. Easy to uninstall?
Control Panel -> Uninstall a program -> uninstall desired benchmark
4. How much time the test will last in heaven
About 3-5 minutes depending on what benchmark you run. Heaven, when running the BENCHMARK...