Add FileHelperTest

package com.fsck.k9.helper

import assertk.assertThat

import assertk.assertions.isEqualTo

import assertk.assertions.isFalse

import assertk.assertions.isNotEqualTo

import assertk.assertions.isTrue

import java.io.File

import kotlin.test.Test

import org.junit.Rule

import org.junit.rules.TemporaryFolder

class FileHelperTest {

    @Rule

    @JvmField

    val tempFolder = TemporaryFolder()

    @Test

    fun `touchFile should create file`() {

        val parent = tempFolder.newFolder("parent")

        FileHelper.touchFile(parent, "FileName")

        assertThat(File(parent, "FileName").exists()).isTrue()

    }

    @Test

    fun `touchFile should change file modification time`() {

        val parent = tempFolder.newFolder("parent")

        val lastModified = 12345L

        val file = File(parent, "FileName")

        file.createNewFile()

        file.setLastModified(lastModified)

        FileHelper.touchFile(parent, "FileName")

        assertThat(file.lastModified()).isNotEqualTo(lastModified)

    }

    @Test

    fun `renameOrMoveByCopying should move file when destination does not exist`() {

        val source = tempFolder.newFile("source")

        val destination = File(tempFolder.root, "destination")

        val sourceContent = "content"

        source.writeText(sourceContent)

        FileHelper.renameOrMoveByCopying(source, destination)

        assertThat(source.exists()).isFalse()

        assertThat(destination.exists()).isTrue()

        assertThat(destination.readText()).isEqualTo(sourceContent)

    }

    @Test

    fun `renameOrMoveByCopying should move file when destination exists`() {

        val source = tempFolder.newFile("source")

        val destination = tempFolder.newFile("destination")

        val sourceContent = "content"

        source.writeText(sourceContent)

        FileHelper.renameOrMoveByCopying(source, destination)

        assertThat(source.exists()).isFalse()

        assertThat(destination.exists()).isTrue()

        assertThat(destination.readText()).isEqualTo(sourceContent)

    }

}