Fix warnings in stdlib/compiler/plugins/test code

compiler/tests-common/tests/org/jetbrains/kotlin/checkers/CheckerTestUtilTest.kt

@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.checkers
 
 import com.google.common.collect.Lists
-import com.intellij.openapi.util.text.StringUtil
 import com.intellij.psi.PsiFile
 import org.jetbrains.kotlin.ObsoleteTestInfrastructure
 import org.jetbrains.kotlin.checkers.diagnostics.ActualDiagnostic
@@ -228,8 +227,8 @@ class CheckerTestUtilTest : KotlinTestWithEnvironment() {
 
         private fun missing(data: DiagnosticData) = missing(data.name, data.startOffset, data.endOffset)
 
-        private fun asTextDiagnostic(diagnosticData: DiagnosticData, vararg params: String) =
-            diagnosticData.name + "(" + StringUtil.join(params, "; ") + ")"
+        private fun asTextDiagnostic(diagnosticData: DiagnosticData, vararg params: String): String =
+            params.joinToString(prefix = diagnosticData.name + "(", postfix = ")", separator = "; ")
 
         private fun asDiagnosticRange(diagnosticData: DiagnosticData, vararg textDiagnostics: String): DiagnosedRange {
             val range = DiagnosedRange(diagnosticData.startOffset)

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

@@ -205,7 +205,7 @@ abstract class AbstractFirDiagnosticsTest : AbstractFirBaseDiagnosticsTest() {
         // Lambda argument is always (?) duplicated by function literal
         // Block expression is always (?) duplicated by single block expression
         if (sourceElement.elementType == KtNodeTypes.LAMBDA_ARGUMENT || sourceElement.elementType == KtNodeTypes.BLOCK) return null
-        val name = name ?: return null
+        val name = name
         if (diagnosedRangesToDiagnosticNames[sourceElement.startOffset..sourceElement.endOffset]?.contains(name) != true) return null
 
         val argumentText = argument()

compiler/tests-common/tests/org/jetbrains/kotlin/ir/AbstractIrTextTestCase.kt

@@ -184,12 +184,10 @@ abstract class AbstractIrTextTestCase : AbstractIrGeneratorTestCase() {
 
         @OptIn(ObsoleteDescriptorBasedAPI::class)
         override fun visitDeclaration(declaration: IrDeclarationBase) {
-            if (declaration is IrSymbolOwner) {
-                declaration.symbol.checkBinding("decl", declaration)
+            declaration.symbol.checkBinding("decl", declaration)
 
-                require(declaration.symbol.owner == declaration) {
-                    "Symbol is not bound to declaration: ${declaration.render()}"
-                }
+            require(declaration.symbol.owner == declaration) {
+                "Symbol is not bound to declaration: ${declaration.render()}"
             }
 
             val containingDeclarationDescriptor = declaration.descriptor.containingDeclaration