to avoid possible 'ignore keywords that exist in more than 50% of rows' trap in MySQL FTS.

that now happens with multiple relation dialog with title.

otherwise full text index is used where keywords that appear in more than 50% of rows are ignored.

so that it supports other comparison operators like '>', 'like' etc.

Amend 9240d498: Since many Loaders are
instanciated by unittest itself, filter_test_list still have to be a class
variable