Update kotlinx-serialization dependency for serialization compiler plugin tests.

Properly set up dom-api-compat dependency for JS IR tests. Since this dependency is added
automatically for every Kotlin/JS library, it should be present during tests just as stdlib.

As a result, tests for serializable enums were changed since 1.6.0 runtime does not require enums to be explicitly serializable.

plugins/kotlinx-serialization/testData/diagnostics/SerializableEnums.kt

@@ -8,7 +8,7 @@ import kotlinx.serialization.encoding.*
 enum class SimpleEnum { A, B }
 
 // Annotated enums do not require @Serializable if runtime has proper factory funciton (runtime ver. >= 1.5.0)
-<!EXPLICIT_SERIALIZABLE_IS_REQUIRED!>enum<!> class MarkedNameEnum { @SerialName("a") A, B}
+enum class MarkedNameEnum { @SerialName("a") A, B}
 
 @Serializable
 enum class ExplicitlyMarkedEnum { @SerialName("a") A, B}