backend/tests: add more interop tests

backend.native/tests/interop/basics/1.kt

@@ -0,0 +1,5 @@
+import cstdlib.*
+
+fun main(args: Array<String>) {
+    println(atoi("257"))
+}

backend.native/tests/interop/basics/2.kt

@@ -0,0 +1,5 @@
+import cstdio.*
+
+fun main(args: Array<String>) {
+    puts("Hello")
+}

backend.native/tests/interop/basics/cstdio.def

@@ -0,0 +1,2 @@
+headers = stdio.h
+excludedFunctions = _IO_flockfile _IO_funlockfile _IO_ftrylockfile _IO_cleanup_region_start _IO_cleanup_region_end