[Test] Explicitly enable UNUSED_* diagnostics in tests which are not belong to contolFlowAnalysis suite

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

@@ -1,8 +1,9 @@
 // !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSICS: +UNUSED_VARIABLE
 
 fun foo(): Any = 42
 
 fun test() {
     when (val <!UNUSED_VARIABLE!>x<!> = foo()) {
     }
-}
+}