Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/classes/initializerBlock.kt

@@ -0,0 +1,13 @@
+class C() {
+  public var f: Int
+
+  {
+    $f = 610
+  }
+}
+
+fun box(): String {
+  val c = C()
+  if (c.f != 610) return "fail"
+  return "OK"
+}