Add basic file content transformations.

^KT-45744 In Progress

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

@@ -0,0 +1,45 @@
+/*
+ * 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
+import java.nio.file.Path
+
+/**
+ * Modify file content under [Path].
+ *
+ * @param transform function receiving current file content and outputting new file content
+ */
+fun Path.modify(transform: (currentContent: String) -> String) {
+    assert(Files.isRegularFile(this)) { "$this is not a regular file!" }
+
+    val file = toFile()
+    file.writeText(transform(file.readText()))
+}
+
+/**
+ * Append [textToAppend] to the file content under [Path].
+ */
+fun Path.append(
+    textToAppend: String
+) {
+    modify {
+        """
+            $it
+            
+            $textToAppend
+        """.trimIndent()
+    }
+}
+
+/**
+ * Append [textToAppend] to the current [TestProject] 'build.gradle' file.
+ */
+fun TestProject.appendToBuildFile(
+    textToAppend: String
+) {
+    projectPath.resolve("build.gradle").append(textToAppend)
+}