Add test to test_files.c

Author: hoodmane

test/test_files.c

@@ -163,6 +163,8 @@ void test_tempfiles() {
   FILE *n = fopen("/dev/null", "w");
   printf("5 bytes to dev/null: %zu\n", fwrite(data, 1, 5, n));
   fclose(n);
+  int res = fseek(n, 10, SEEK_CUR);
+  printf("seek /dev/null: %d\n", res);
 
   // Test file creation with O_TRUNC (regression test for #16784)
   const char *file_name = "test.out";

test/test_files.out

@@ -12,4 +12,5 @@ seeked=ata.
 seeked=ta.
 fscanfed: 10 - hello
 5 bytes to dev/null: 5
+seek /dev/null: 0
 ok.