Modify file exists assertion.

Additionally allow to check if file exists in any location, even outside
of the project.

^KT-45745 In Progress

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

@@ -20,7 +20,7 @@ class SimpleKotlinGradleIT : KGPBaseTest() {
         ) {
             build("compileDeployKotlin", "build") {
                 assertOutputContains("Finished executing kotlin compiler using daemon strategy")
-                assertFileExists("build/reports/tests/test/classes/demo.TestSource.html")
+                assertFileInProjectExists("build/reports/tests/test/classes/demo.TestSource.html")
                 assertTasksExecuted(":compileKotlin", ":compileTestKotlin", ":compileDeployKotlin")
             }
 
@@ -87,7 +87,7 @@ class SimpleKotlinGradleIT : KGPBaseTest() {
     fun testModuleName(gradleVersion: GradleVersion) {
         project("moduleName", gradleVersion) {
             build("build") {
-                assertFileExists("build/classes/kotlin/main/META-INF/FLAG.kotlin_module")
+                assertFileInProjectExists("build/classes/kotlin/main/META-INF/FLAG.kotlin_module")
                 assertFileNotExists("build/classes/kotlin/main/META-INF/moduleName.kotlin_module")
                 assertOutputDoesNotContain("Argument -module-name is passed multiple times")
             }

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

@@ -6,21 +6,30 @@
 package org.jetbrains.kotlin.gradle.testbase
 
 import java.nio.file.Files
+import java.nio.file.Path
+
+/**
+ * Asserts file under [file] path exists and is a regular file.
+ */
+fun TestProject.assertFileExists(
+    file: Path
+) {
+    assert(Files.exists(file)) {
+        "File '${file}' does not exist!"
+    }
+
+    assert(Files.isRegularFile(file)) {
+        "'${file}' is not a regular file!"
+    }
+}
 
 /**
  * Asserts file under [pathToFile] relative to the test project exists and is a regular file.
  */
-fun TestProject.assertFileExists(
+fun TestProject.assertFileInProjectExists(
     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!"
-    }
+    assertFileExists(projectPath.resolve(pathToFile))
 }
 
 /**