Drop single-file mode of black box codegen tests

Add regression test for #KT-5190

compiler/testData/codegen/java8/box/useStream.kt

@@ -0,0 +1,13 @@
+// KT-5190 Java 8 Stream.collect couldn't be called
+// FULL_JDK
+
+import java.util.stream.Collectors
+import java.util.stream.Stream
+
+fun box(): String {
+    val mutableListOf = mutableListOf("OK", "B", "C")
+
+    return test((mutableListOf as java.util.Collection<String>).stream()) as String
+}
+
+fun test(a: Stream<String>) = a.collect(Collectors.toList()).first()