[FIR] extended checkers infrastructure refactoring

Removed strange way to enable Extended Checkers and add new way to do it

compiler/tests-common/tests/org/jetbrains/kotlin/fir/AbstractExtendedFirDiagnosticsTest.kt

@@ -5,6 +5,11 @@
 
 package org.jetbrains.kotlin.fir
 
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.ExtendedDeclarationCheckers
+import org.jetbrains.kotlin.fir.analysis.checkersComponent
+
 abstract class AbstractExtendedFirDiagnosticsTest : AbstractFirDiagnosticsTest() {
-    override fun useExtendedCheckersIfNeeded(): Boolean = true
+    override fun configureSession(session: FirSession) {
+        session.checkersComponent.register(ExtendedDeclarationCheckers)
+    }
 }

compiler/tests-common/tests/org/jetbrains/kotlin/fir/AbstractFirDiagnosticTest.kt

@@ -276,7 +276,7 @@ abstract class AbstractFirDiagnosticsTest : AbstractFirBaseDiagnosticsTest() {
     }
 
     private fun createCollector(session: FirSession): AbstractDiagnosticCollector {
-        return FirDiagnosticsCollector.create(session, useExtendedCheckersIfNeeded())
+        return FirDiagnosticsCollector.create(session)
     }
 
     private fun checkCfgDump(testDataFile: File, firFiles: List<FirFile>) {
@@ -360,5 +360,4 @@ abstract class AbstractFirDiagnosticsTest : AbstractFirBaseDiagnosticsTest() {
 
     }
 
-    internal open fun useExtendedCheckersIfNeeded() = false
 }