[K/N][Tests] Adjust moved codegen tests to new infra

^KT-61259

native/native.tests/testData/codegen/contracts/isNullString.kt

@@ -0,0 +1,21 @@
+import kotlin.test.*
+import kotlin.contracts.*
+
+open class S
+class P(val str: String = "P") : S()
+
+@OptIn(kotlin.contracts.ExperimentalContracts::class)
+fun check(actual: Boolean) {
+    contract { returns() implies actual }
+    assertTrue(actual)
+}
+
+fun box(): String {
+    assertEquals("STR", nullableString("str"))
+
+    assertEquals("", nullableString(null))
+
+    return "OK"
+}
+
+private fun nullableString(string: String?): String = if (string.isNullOrBlank()) "" else "STR"