[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/when/withSubjectVariable/unsupportedFeature.fir.kt

@@ -0,0 +1,16 @@
+// !LANGUAGE: -VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+fun foo(): Any = 42
+
+fun test(x: Any) {
+    // NB check that we still resolve 'y', even though current language version doesn' support variable declaration in when subject
+
+    val z1 = when (val y = foo()) {
+        42 -> "Magic: $y, $x"
+        else -> {
+            "Not magic: $y, $x"
+        }
+    }
+    val z2 = "Anyway, it was $<!UNRESOLVED_REFERENCE!>y<!>"
+}