Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/boxingOptimization/nullCheck.kt

@@ -0,0 +1,17 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+inline fun <R, T> foo(x : R?, y : R?, block : (R?) -> T) : T {
+    if (x == null) {
+        return block(x)
+    } else {
+        return block(y)
+    }
+}
+
+fun box() : String {
+    assertEquals(3, foo(1, 2) { x -> if (x != null) 3 else 4 })
+
+    return "OK"
+}