[tests] add new custom extension 'reversed' for tests

the new test will be introduced in the next commits

^KT-55750

compiler/test-infrastructure-utils/tests/org/jetbrains/kotlin/test/utils/CustomTestDataUtils.kt

@@ -14,7 +14,7 @@ private const val KTS = ".kts"
 private const val FIR_PREFIX = ".fir"
 private const val LL_FIR_PREFIX = ".ll"
 
-const val CUSTOM_TEST_DATA_EXTENSION_PATTERN = "^(.+)\\.(fir|ll)\\.kts?\$"
+const val CUSTOM_TEST_DATA_EXTENSION_PATTERN = "^(.+)\\.(reversed|fir|ll)\\.kts?\$"
 
 val File.isFirTestData: Boolean
     get() = isCustomTestDataWithPrefix(FIR_PREFIX)