[K/N] Move special backend checks test sources to diagnostic tests folder

^KT-61564

compiler/testData/diagnostics/nativeTests/specialBackendChecks/cInterop/t38.kt

@@ -0,0 +1,14 @@
+import kotlinx.cinterop.*
+import kotlinx.cinterop.internal.*
+
+@CStruct(spelling = "struct { }") class Z constructor(rawPtr: NativePtr) : CStructVar(rawPtr) {
+    var x: Pair<Int, Int>? = null
+        @CStruct.MemberAt(offset = 0L) get
+        @CStruct.MemberAt(offset = 0L) set
+}
+
+fun foo(z: Z) {
+    z.x = 42 to 117
+}
+
+fun main() { }