database: increase size of TKO's test attributes value

Turns out that the definition of the test attribute value size set in the tko models file (frontend/tko/models.py) was a lot smaller than the size on the SQL schema file:

schema.sql snippet:

```
   CREATE TABLE `tko_test_attributes` (
      `value` varchar(1024) DEFAULT NULL,
```

frontent/tko/models.py snippet:

```
   class TestAttribute(dbmodels.Model, model_logic.ModelExtensions):
       value = dbmodels.CharField(blank=True, max_length=300)
```

This has been the case since 2009, according to commit f756779f.

So, increase the size so that attributes with larger values do not get truncated.

Signed-off-by: Cleber Rosa <crosa@redhat.com>