Mark expect/actual classifiers as experimental

^KT-61573 Fixed
Review: https://jetbrains.team/p/kt/reviews/11969/timeline

Tests:
- MultiPlatformIntegrationTestGenerated
- CliTestGenerated
- MultiPlatformIntegrationTestGenerated
- DiagnosticTestGenerated.Multiplatform
- FirLightTreeOldFrontendDiagnosticsTestGenerated

Also add -Xexpect-actual-classes flag to all necessary ./libraries/* modules
Otherwise compilation of those modules failes because of `-Werror`

libraries/kotlin.test/junit/build.gradle

@@ -32,6 +32,7 @@ RepoArtifacts.javadocJar(project)
 compileKotlin {
     kotlinOptions.freeCompilerArgs = [
         "-Xallow-kotlin-package",
+        "-Xexpect-actual-classes",
     ]
     kotlinOptions.moduleName = project.name
 }

libraries/kotlin.test/junit5/build.gradle

@@ -33,6 +33,7 @@ RepoArtifacts.javadocJar(project)
 compileKotlin {
     kotlinOptions.freeCompilerArgs = [
         "-Xallow-kotlin-package",
+        "-Xexpect-actual-classes",
     ]
     kotlinOptions.moduleName = project.name
 }

libraries/kotlin.test/testng/build.gradle

@@ -34,6 +34,7 @@ RepoArtifacts.javadocJar(project)
 compileKotlin {
     kotlinOptions.freeCompilerArgs = [
         "-Xallow-kotlin-package",
+        "-Xexpect-actual-classes",
     ]
     kotlinOptions.moduleName = project.name
 }