tap13: fix worker fail on failed TAP13 parsing

The error is the following.

```
[050] unit/mp_tuple.test
[050] TAP13 parse failed (Missing plan in the TAP source).
[050]
[050] No result file (unit/mp_tuple.result) found.
[050] Run the test with --update-result option to write the new result file.
[050]
[050] Worker "050_unit" received the following error; stopping...
[050] Traceback (most recent call last):
[050]   File "/__w/tarantool/tarantool/test-run/lib/worker.py", line 347, in run_task
[050]     short_status, duration = self.suite.run_test(
[050]   File "/__w/tarantool/tarantool/test-run/lib/test_suite.py", line 277, in run_test
[050]     short_status = test.run(server)
[050]   File "/__w/tarantool/tarantool/test-run/lib/test.py", line 233, in run
[050]     is_tap, is_ok, is_skip = self.check_tap_output()
[050] ValueError: not enough values to unpack (expected 3, got 2)
[050] Exception: not enough values to unpack (expected 3, got 2)
```

The worker failure leads to the following error at end of the testing.

> [Internal test-run error] The following tasks were dispatched to some
> worker task queue, but were not reported as done (does not matters
> success or fail)

It is fixed now.

Follows up #381
Fixes #414

lib/test.py

@@ -363,7 +363,7 @@ def check_tap_output(self):
                 msg = 'Run the test with --update-result option to write the new result file.\n'
                 color_stdout(msg, schema='error')
             self.is_crash_reported = True
-            return False, False
+            return False, False, False
 
         is_ok = True
         is_skip = False