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

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

@@ -0,0 +1,8 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun foo(): Any = 42
+
+fun test() {
+    when (val x = foo()) {
+    }
+}