Use pledge(2), the sanity test does even not read files.

Index: tests/sanity_test.c
--- tests/sanity_test.c.orig
+++ tests/sanity_test.c
@@ -639,6 +639,13 @@ int main(int argc, const char* argv[])
     (void) argc;
     (void) argv;
 
+#ifdef __OpenBSD__
+    if (pledge("stdio", NULL) == -1) {
+        XSUM_log("pledge: %s\n", strerror(errno));
+        return EXIT_FAILURE;
+    }
+#endif
+
     {
         /* XXH32 */
         size_t const n = sizeof(XSUM_XXH32_testdata) / sizeof(XSUM_XXH32_testdata[0]);