nitunit: accept an absolute path for `--dir`.

When I passed an absolute path to the `--dir` option of `nitunit`, I got errors like this one:
```
ERROR: test_unread_order (in file /my/costum/dir/.nitunit/test_push_back_reader_TestPushBackDecorator_test_unread_order.nit): sh: 1: .//my/costum/dir/.nitunit/test_push_back_reader_TestPushBackDecorator_test_unread_order.bin: not found
```

It was a trivial bug so I fixed this.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
`has_prefix`?