[Gradle] Add unit tests for mergeWith

^KT-49933

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/utils/mergeWithTest.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle.utils
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class MergeWithTest {
+    private fun Map<String, Set<Int>>.prettyStringForDiff() = entries
+        .sortedBy { it.key }
+        .joinToString("\n") { (key, setOfInts) -> "$key => ${setOfInts.sorted()}" }
+
+    private fun assertEquals(expected: Map<String, Set<Int>>, actual: Map<String, Set<Int>>) {
+        assertEquals(expected.prettyStringForDiff(), actual.prettyStringForDiff())
+    }
+
+    private val sample = mapOf(
+        "a" to setOf(1, 2),
+        "b" to setOf(3, 4),
+        "c" to emptySet(),
+    )
+
+
+    @Test
+    fun basicTest() {
+        val a = mapOf(
+            "a" to setOf(1, 2),
+            "b" to setOf(3, 4)
+        )
+
+        val b = mapOf(
+            "b" to setOf(3, 5),
+            "c" to setOf(6),
+            "d" to emptySet(),
+        )
+
+        val actual = a mergeWith b
+        val expected = mapOf(
+            "a" to setOf(1, 2),
+            "b" to setOf(3, 4, 5),
+            "c" to setOf(6),
+            "d" to emptySet()
+        )
+
+        assertEquals(expected, actual)
+    }
+
+    @Test
+    fun mergeWithSelf() = assertEquals(sample, sample mergeWith sample)
+
+    @Test
+    fun mergeWithEmpty() {
+        assertEquals(sample, emptyMap<String, Set<Int>>() mergeWith sample)
+        assertEquals(sample, sample mergeWith emptyMap())
+        assertEquals(emptyMap(), emptyMap<String, Set<Int>>() mergeWith emptyMap())
+    }
+}