Fix stray output with out-of-order error

[XavierCooney]

A missing comma led to implicit string concatenation in the error message for when the output lines are correct but in the wrong order:

$ cat tests.txt
files=prog.c
1 expected_stdout='1\n2\n3\n'
$ cat prog.c
#include <stdio.h>
int main(void) {
    printf("3\n2\n1\n");
}
$ autotest -a .
[...]
Test 1 (prog) - failed (Incorrect output)

Your program produced the correct output lines but in the wrong order.
redYour program produced these 3 lines of output:
[...]