test: thumbnail: fix spurious fail

Fixes the following assert:
```
Assertion failed: (status == -EINTR), function thumbnailer_callback_cancel, file thumbnail.c, line 194.
FAIL test_src_preparser_thumbnail (exit status: 134)
```

By increasing the length of the mock used to test cancellation. It was
200us, so it was possible that the thumbnailer finished before we could
cancel it.

Note that can_control_pace is false and that there are no video tracks,
so the thumbnailer will take 20 secs to find a thumbnail (and fail),
giving us enough time to cancel it.

Fixes #29018