Add Job preference tests

[ElectroJr]

The main point of this PR is to add several job related tests that check that preferences, weights, and priorities work. See JobTest.cs

Other Changes:

• Adds methods to TestPair.Helpers.cs for modifying a job preferences
• Test pairs now automatically reset any modified preferences when returning to the pool.
• Replaced various strings with ProtoId<JobPrototype> or ProtoId<AntagPrototype>.
• Fixes some HumanoidCharacterProfile methods to ensure that only a single job can be set to high priority
• Changed StationJobsComponent:
  • The MidRoundTotalJobs, OverflowJobs and RoundStartJobList fields are no longer datafields, as they can just be inferred from the SetupAvailableJobs field.
  • The "JobList" dictionary now uses int? instead of uint? values. They're still always positive, it just reduced the amount of annoying casts.