Add test for KT-21710 (gutters inside library sources)

idea/testData/codeInsightInLibrary/_library/InterfaceClass.kt

@@ -0,0 +1,16 @@
+package library
+
+interface <lineMarker descr="*">My</lineMarker> {
+    fun <lineMarker descr="*">foo</lineMarker>(): Int
+
+    val <lineMarker descr="*">s</lineMarker>: String
+}
+
+class Your : My {
+    override fun <lineMarker descr="*">foo</lineMarker>(): Int {
+        return 42
+    }
+
+    override val <lineMarker descr="*">s</lineMarker>: String
+        get() = ""
+}

idea/testData/codeInsightInLibrary/lineMarker/dummy.kt

@@ -0,0 +1,3 @@
+fun <lineMarker>main</lineMarker>(args: Array<String>) {
+
+}