Test infra: refactor IGNORE_BACKEND directive

treat it as a general one, introduce *_K1 and *_K2 variants for
more specific ignoring

compiler/testData/codegen/box/defaultArguments/useNextParamInLambdaTailrec.kt

@@ -1,5 +1,5 @@
 // IGNORE_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature. UNINITIALIZED_PARAMETER y. See KT-49800
 tailrec fun foo(x: () -> String? = { y }, y: String = "fail"): String? {
     if (y == "start")