Added functional test on Gradle nested classes validation error

Resolves #437

PR #449

kover-gradle-plugin/src/functionalTest/kotlin/kotlinx/kover/gradle/plugin/test/functional/cases/GradleValidationTests.kt

@@ -0,0 +1,16 @@
+package kotlinx.kover.gradle.plugin.test.functional.cases
+
+import kotlinx.kover.gradle.plugin.test.functional.framework.checker.CheckerContext
+import kotlinx.kover.gradle.plugin.test.functional.framework.starter.TemplateTest
+import kotlin.test.assertFalse
+
+internal class GradleValidationTests {
+
+    // test on validation error /~https://github.com/gradle/gradle/issues/26018
+    @TemplateTest("counters", [":koverXmlReport"])
+    fun CheckerContext.testNestedTypes() {
+        assertFalse("Nested classes validation error. \n Build log:\n $output") {
+            output.contains("Nested types are expected to either declare some annotated properties or some behaviour that requires capturing the type as input")
+        }
+    }
+}