FIR IDE: Refactor AbstractFirShortenRefsTest

- Use `FIR_COMPARISON` directive
- Move testing utils to `ideaFirTestUtils.kt`
- Ignore `TopLevelFunctionImportWithLotsOfFqName.kt` test for now (it
takes too long to execute)

idea/idea-fir/tests/org/jetbrains/kotlin/idea/ideaFirTestUtils.kt

@@ -17,6 +17,8 @@ import org.jetbrains.kotlin.idea.frontend.api.fir.KtFirAnalysisSessionProvider
 import org.jetbrains.kotlin.psi.KtElement
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.trackers.KotlinOutOfBlockModificationTrackerFactory
+import org.jetbrains.kotlin.test.InTextDirectivesUtils
+import java.io.File
 
 fun Throwable.shouldBeRethrown(): Boolean = when (this) {
     is DuplicatedFirSourceElementsException -> true