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/forEachFloatArray.kt

@@ -0,0 +1,6 @@
+fun box(): String {
+    for (x in FloatArray(5)) {
+        if (x != 0.toFloat()) return "Fail $x"
+    }
+    return "OK"
+}