[ObjCExport] Extract generation of ObjC base declarations into shared code

KT-64226

native/objcexport-header-generator/test/org/jetbrains/kotlin/backend/konan/testUtils/testDataDir.kt

@@ -8,4 +8,5 @@ package org.jetbrains.kotlin.backend.konan.testUtils
 import java.io.File
 
 val testDataDir = File("native/objcexport-header-generator/testData")
-val headersTestDataDir = testDataDir.resolve("headers")
+val headersTestDataDir = testDataDir.resolve("headers")
+val baseDeclarationsDir = testDataDir.resolve("baseDeclarations")

native/objcexport-header-generator/test/org/jetbrains/kotlin/backend/konan/tests/ObjCExportBaseDeclarationsTest.kt

@@ -0,0 +1,50 @@
+/*
+ * 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.backend.konan.tests
+
+import org.jetbrains.kotlin.backend.konan.objcexport.ObjCTopLevel
+import org.jetbrains.kotlin.backend.konan.objcexport.StubRenderer
+import org.jetbrains.kotlin.backend.konan.testUtils.baseDeclarationsDir
+import org.jetbrains.kotlin.test.KotlinTestUtils
+import org.junit.jupiter.api.Test
+import java.io.File
+
+/**
+ * ## Test Scope
+ * This test will just invoke the generation of 'base declarations' (basically ObjC stubs that shall always be present)
+ * The generated declarations will be compared to previously checked-in 'golden' headers.
+ */
+class ObjCExportBaseDeclarationsTest(
+    /**
+     * Injected implementation: Either K1 or based upon Analysis API.
+     */
+    private val generator: BaseDeclarationsGenerator,
+) {
+
+    @Test
+    fun `test - noTopLevelPrefix`() {
+        doTest(baseDeclarationsDir.resolve("!noTopLevelPrefix.h"), "")
+    }
+
+    @Test
+    fun `test - topLevelPrefix`() {
+        doTest(baseDeclarationsDir.resolve("!topLevelPrefix.h"), "MyTopLevelPrefix")
+    }
+
+    private fun doTest(headerFile: File, topLevelPrefix: String) {
+        val declarations = generator(topLevelPrefix)
+
+        val renderedDeclarations = declarations
+            .flatMap { declaration -> StubRenderer.render(declaration) }
+            .joinToString(System.lineSeparator())
+
+        KotlinTestUtils.assertEqualsToFile(headerFile, renderedDeclarations)
+    }
+
+    fun interface BaseDeclarationsGenerator {
+        operator fun invoke(topLevelPrefix: String): List<ObjCTopLevel>
+    }
+}