Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/arrays/arrayPlusAssign.kt

@@ -0,0 +1,6 @@
+fun box(): String {
+    val s = IntArray(1)
+    s[0] = 5
+    s[0] += 7
+    return if (s[0] == 12) "OK" else "Fail ${s[0]}"
+}