Draft: tests: set milestone.deadline_unix in fixtures
PR: https://github.com/go-gitea/gitea/pull/18237 (merged)
The value of deadline_unix must be set to 253370764800 (i.e. 9999-01-01) in fixtures, otherwise it will be inserted as null which leads to unexpected errors. For instance, DumpRepository will store a null deadline_unix as 0 (i.e. 1970-01-01) and RestoreRepository will change it to 9999-01-01.