[JS] set LV=1.9 for K1-specific IC tests

Otherwise they will actully run K2 after
the LV=2.0 migration.

^KT-60829 fixed

compiler/incremental-compilation-impl/test/org/jetbrains/kotlin/incremental/AbstractIncrementalJsCompilerRunnerTest.kt

@@ -44,6 +44,7 @@ abstract class AbstractIncrementalJsCompilerRunnerTest : AbstractIncrementalComp
             sourceMap = true
             metaInfo = true
             forceDeprecatedLegacyCompilerUsage = true
+            languageVersion = "1.9"
         }
 
     protected open val scopeExpansionMode = CompileScopeExpansionMode.NEVER