If an employee has symptoms and is advised by Public Health to self quarantine but is not tested, does the employer have an obligation to inform that employee’s co-workers?

No. There are many possible scenarios. It may be wise to engage the services of an occupational health expert to address individual cases to ask for detailed medical specifics in strict confidence so as to advise the employer of appropriate steps to be taken in the workplace or regarding the eventual return to work of the employee.