test: Begin test about shift inventory animation

src/test/kotlin/com/github/rushyverse/api/gui/load/ShiftInventoryLoadingAnimationTest.kt

@@ -0,0 +1,71 @@
+package com.github.rushyverse.api.gui.load
+
+import io.kotest.matchers.collections.shouldContainExactly
+import io.kotest.matchers.shouldBe
+import io.mockk.every
+import io.mockk.mockk
+import kotlin.test.BeforeTest
+import kotlin.test.Test
+import kotlin.time.Duration.Companion.milliseconds
+import kotlinx.coroutines.cancelAndJoin
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.runBlocking
+import org.bukkit.inventory.Inventory
+import org.bukkit.inventory.ItemStack
+
+class ShiftInventoryLoadingAnimationTest {
+
+    private lateinit var inventory: Inventory
+
+    @BeforeTest
+    fun onBefore() {
+        inventory = mockk {
+            val contents = arrayOfNulls<ItemStack>(9 * 3)
+            every { size } returns contents.size
+            every { getContents() } returns contents
+            every { setContents(any()) } answers {
+                val items = firstArg<Array<ItemStack?>>()
+                items.forEachIndexed { index, itemStack ->
+                    contents[index] = itemStack
+                }
+        }
+            }
+    }
+
+    @Test
+    fun `should not change inventory if initialize is empty`() {
+        val delay = 10.milliseconds
+        val animation = ShiftInventoryLoadingAnimation<Unit>(
+            initialize = { emptySequence() },
+            shift = 1,
+            delay = delay
+        )
+
+        runBlocking {
+            val job = launch { animation.loading(Unit, inventory) }
+            delay(delay * 3)
+            job.cancelAndJoin()
+        }
+
+        inventory.contents shouldBe arrayOfNulls(inventory.size)
+    }
+
+    @Test
+    fun `should just initialize if shift is zero`() = runBlocking {
+        val delay = 10.milliseconds
+        val items = Array(inventory.size) { mockk<ItemStack>() }
+
+        val animation = ShiftInventoryLoadingAnimation<Unit>(
+            initialize = { items.asSequence() },
+            shift = 0,
+            delay = delay
+        )
+
+        val job = launch { animation.loading(Unit, inventory) }
+        delay(delay * 3)
+        job.cancelAndJoin()
+
+        inventory.contents.toList() shouldContainExactly items.toList()
+    }
+}