Generate Unit Test for the following code using JUnit:
```kt
class Solution {
    fun twoSum(nums: IntArray, target: Int): IntArray {
        val lookup = HashMap<Int, Int>()
        nums.forEachIndexed { i, x ->
            val j = lookup[target - x]
            j?.let {
                return intArrayOf(j, i)
            }
            lookup[x] = i
        }
        return intArrayOf()
    }
}
``` Limit your response to 600 words.
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