FIR: mute a blackbox test that fails due to another case of #3747

compiler/testData/codegen/box/invokedynamic/sam/functionRefToJavaInterface/specialFunctions/intPlus.kt

@@ -1,6 +1,9 @@
 // TARGET_BACKEND: JVM
 // IGNORE_BACKEND: ANDROID
 //  ^ uses API not implemented on minSdkVersion 19
+// IGNORE_BACKEND_FIR: JVM_IR
+//  ^ fir2ir creates an IrConst(value=2, type=@EnhancedNullability Int), which
+//    the backend generates as an int instead of Integer
 // WITH_RUNTIME
 // JVM_TARGET: 1.8
 // FULL_JDK