detect/test: update test for file prune changes

As the file prune is now moved to the flow worker, the file
prune is run later, meaning the first file has not yet
been pruned from the file container list.

Adjust test to look for a second file, and check the
flags on that file.

For commit addressing bug 2490.

src/detect-engine-state.c

@@ -1107,6 +1107,8 @@ static int DeStateSigTest08(void)
     FAIL_IF_NULL(files);
     file = files->head;
     FAIL_IF_NULL(file);
+    file = file->next;
+    FAIL_IF_NULL(file);
     FAIL_IF_NOT(file->flags & FILE_STORE);
 
     AppLayerParserThreadCtxFree(alp_tctx);