Move common logic from CodegenTestCase to KotlinBaseTest

plugins/kapt3/kapt3-compiler/test/org/jetbrains/kotlin/kapt3/test/AbstractIrKotlinKapt3IntegrationTest.kt

@@ -8,5 +8,5 @@ package org.jetbrains.kotlin.kapt3.test
 import org.jetbrains.kotlin.test.TargetBackend
 
 abstract class AbstractIrKotlinKapt3IntegrationTest : AbstractKotlinKapt3IntegrationTest() {
-    override fun getBackend() = TargetBackend.JVM_IR
+    override val backend = TargetBackend.JVM_IR
 }

plugins/kapt3/kapt3-compiler/test/org/jetbrains/kotlin/kapt3/test/AbstractIrKotlinKapt3Test.kt

@@ -12,9 +12,9 @@ Currently Kapt3 only works with the old backend. To enable IR, modify the isIrBa
 */
 
 abstract class AbstractIrClassFileToSourceStubConverterTest : AbstractClassFileToSourceStubConverterTest() {
-    override fun getBackend() = TargetBackend.JVM_IR
+    override val backend = TargetBackend.JVM_IR
 }
 
 abstract class AbstractIrKotlinKaptContextTest : AbstractKotlinKaptContextTest() {
-    override fun getBackend() = TargetBackend.JVM_IR
+    override val backend = TargetBackend.JVM_IR
 }