Added test for PrimeManager::selectOnlyPrimes

Method returns array in unexpected format, like
`array(0 => 2, 1 => 3, 3 => 5,...)` but expected `array(0 => 2, 1 => 3, 2 => 5,...)`
Because of that, tests which has multiple elements with non-prime numbers always fail.
Also test for negative numbers and zero fails.