Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/controlStructures/compareBoxedIntegerToZero.kt

@@ -0,0 +1,8 @@
+fun box(): String {
+    val x: Int? = 0
+    if (x != 0) return "Fail $x"
+    if (0 != x) return "Fail $x"
+    if (!(x == 0)) return "Fail $x"
+    if (!(0 == x)) return "Fail $x"
+    return "OK"
+}