add public method for htmlHierarchy saving

[nizienko]

we already have an example:

    private fun saveHierarchy(testName: String) {
        val hierarchySnapshot =
            saveFile(url, "build/reports", "hierarchy-$testName.html")
        if (File("build/reports/styles.css").exists().not()) {
            saveFile("$url/styles.css", "build/reports", "styles.css")
        }
        println("Hierarchy snapshot: ${hierarchySnapshot.absolutePath}")
    }

    private fun saveFile(url: String, folder: String, name: String): File {
        val response = client.newCall(Request.Builder().url(url).build()).execute()
        return File(folder).apply {
            mkdirs()
        }.resolve(name).apply {
            writeText(response.body?.string() ?: "")
        }
    }