Add file or directory exists/does not exist assertions.

^KT-45744 In Progress

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/SimpleKotlinGradleIT.kt

@@ -1,5 +1,4 @@
 package org.jetbrains.kotlin.gradle
-
 import org.gradle.api.logging.configuration.WarningMode
 import org.jetbrains.kotlin.gradle.util.modify
 import org.junit.Test

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/testbase/fileAssertions.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle.testbase
+
+import java.nio.file.Files
+
+/**
+ * Asserts file under [pathToFile] relative to the test project exists and is a regular file.
+ */
+fun TestProject.assertFileExists(
+    pathToFile: String
+) {
+    val filePath = projectPath.resolve(pathToFile)
+    assert(Files.exists(filePath)) {
+        "File '${filePath}' does not exist!"
+    }
+
+    assert(Files.isRegularFile(filePath)) {
+        "'${filePath}' is not a regular file!"
+    }
+}
+
+/**
+ * Asserts file under [pathToFile] relative to the test project does not exist.
+ */
+fun TestProject.assertFileNotExists(
+    pathToFile: String
+) {
+    val filePath = projectPath.resolve(pathToFile)
+    assert(!Files.exists(filePath)) {
+        "File '${filePath}' exists!"
+    }
+}
+
+/**
+ * Asserts directory under [pathToDir] relative to the test project exists and is a directory.
+ */
+fun TestProject.assertDirectoryExists(
+    pathToDir: String
+) {
+    val dirPath = projectPath.resolve(pathToDir)
+
+    assert(Files.exists(dirPath)) {
+        "Directory '${dirPath}' does not exist!"
+    }
+
+    assert(Files.isDirectory(dirPath)) {
+        "'${dirPath}' is not a directory!"
+    }
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/testbase/testDsl.kt

@@ -37,7 +37,8 @@ fun KGPBaseTest.project(
     val testProject = TestProject(
         gradleRunner,
         projectName,
-        buildOptions
+        buildOptions,
+        projectPath
     )
     testProject.test()
     return testProject
@@ -74,7 +75,8 @@ fun TestProject.buildAndFail(
 class TestProject(
     val gradleRunner: GradleRunner,
     val projectName: String,
-    val buildOptions: KGPBaseTest.BuildOptions
+    val buildOptions: KGPBaseTest.BuildOptions,
+    val projectPath: Path
 )
 
 private fun TestProject.commonBuildSetup(