FIR IDE: fix failing AbstractOverrideImplementTest tests on Windows

Before the System.lineSeparator() was used for splitting KtFile.text into lines.
In IJ all lines has UNIX file endings, so it failed on Windows TC agent

idea/tests/org/jetbrains/kotlin/idea/codeInsight/AbstractOverrideImplementTest.kt

@@ -204,7 +204,7 @@ abstract class AbstractOverrideImplementTest<T : ClassMember> : KotlinLightCodeI
             Assert.assertTrue(resultFile.exists())
             val errorLines = myFixture.dumpErrorLines()
             val file = myFixture.file as KtFile
-            val fileLines = file.text.split(System.lineSeparator())
+            val fileLines = file.text.lines()
             val newTextContent = if (firIdenticalIsPresent) {
                 // Ensure the directive is the first line
                 val fileLinesWithoutFirIdentical = fileLines.filter { it != IgnoreTests.DIRECTIVES.FIR_IDENTICAL }