tests/example.c: Use errno = ENOMEM.

The output from errno = 0 is less portable.

tests/example.c

@@ -32,7 +32,7 @@ int main(int argc, char **argv unused) {
     printf("write to stdout\n");
     fflush(stdout);
 
-    errno = 0;
+    errno = ENOMEM;
     perror("error!");
 
     write(STDERR_FILENO, "write to stderr 2", 17);

tests/example.expected

@@ -1,6 +1,6 @@
 >STDERR>write to stderr: 1
 <STDERR<write to stdout
->STDERR>error!: Success
+>STDERR>error!: Cannot allocate memory
 <STDERR<>STDERR>write to stderr 2<STDERR<write to stdout 2>STDERR>
 <STDERR<
 >STDERR>more on stderr

tests/example_environment_empty.expected

@@ -1,6 +1,6 @@
 write to stderr: 1
 write to stdout
-error!: Success
+error!: Cannot allocate memory
 write to stderr 2write to stdout 2
 
 more on stderr