tests: More work on error handling

tests/unistd/link.c

@@ -7,7 +7,7 @@
 #include "test_helpers.h"
 
 int main(void) {
-    printf("%ld\n", sizeof(struct stat));
+    printf("sizeof(struct stat): %ld\n", sizeof(struct stat));
 
     struct stat buf;
 

tests/unistd/write.c

@@ -3,5 +3,6 @@
 #include "test_helpers.h"
 
 int main(void) {
-    write(STDOUT_FILENO, "Hello World!\n", 13);
+    int written = write(STDOUT_FILENO, "Hello World!\n", 13);
+    ERROR_IF(write, written, == -1);
 }