InstantMockKotlin.kts

import org.junit.jupiter.api.Assertions
import org.junit.jupiter.api.Test
import java.time.Instant

class MyObject(
    private val currentInstant: () -> Instant = { Instant.now() }
) {

    fun myMethod() = Pair("test", currentInstant())

}

class MyObjectTest {

    @Test
    fun `my test`() {
        val testInstant = Instant.ofEpochSecond(100)

        val underTest = MyObject({ testInstant })

        Assertions.assertEquals(
            Pair("test", testInstant),
            underTest.myMethod()
        )
    }
}