Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/extensionFunctions/kt1953_class.kt

@@ -0,0 +1,14 @@
+class A {
+    private val sb: StringBuilder = StringBuilder()
+
+    fun String.plus() {
+        sb.append(this)
+    }
+
+    fun foo(): String {
+        +"OK"
+        return sb.toString()!!
+    }
+}
+
+fun box(): String = A().foo()