ktest: print tested output file in case of a match failure (fixes #689)

Changed error reporting when an output mismatch happened from this:

```
ERROR: ['/home/omer/K/k_new_dist/k/bin/krun' '/home/omer/K/k_new_dist/k/tutorial/1_k/5_types/lesson_4/tests/composition.lambda' '--search' '--pattern' '<k> V:K </k>' '--directory' '/home/omer/K/k_new_dist/k/tutorial/1_k/5_types/lesson_9'] output doesn't match with expected output (time: 11072 ms)
```

to this:

```
ERROR: ['/home/omer/K/k_new_dist/k/bin/krun' '/home/omer/K/k_new_dist/k/tutorial/1_k/5_types/lesson_4/tests/composition.lambda' '--search' '--pattern' '<k> V:K </k>' '--directory' '/home/omer/K/k_new_dist/k/tutorial/1_k/5_types/lesson_9'] output doesn't match with expected output /home/omer/K/k_new_dist/k/tutorial/1_k/5_types/lesson_4/tests/composition.lambda.out (time: 11578 ms)
 ```

(not the extra file path that points to compared output file)

#689