[LL FIR] rename testdata to testData to avoid copyright generation

We exclude testData pattern from copyright scope

analysis/low-level-api-fir/testData/fileStructure/nestedClasesWithFun.kt

@@ -0,0 +1,7 @@
+class A {/* ClassDeclarationStructureElement */
+    fun y() {/* DeclarationStructureElement */}
+
+    class B {/* ClassDeclarationStructureElement */
+        fun x() {/* DeclarationStructureElement */}
+    }
+}