Add test for UNUSED_VALUE on when subject variable

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

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

compiler/testData/diagnostics/tests/when/withSubjectVariable/unusedWhenSubjectVariable.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(): kotlin.Any
+public fun test(): kotlin.Unit