Presuming that you won't be testing a date which is a Saturday then:
WeekEndDate = TestingDate + 6 - Weekday(TestingDate)
This is because weekday function returns number between 1-7 depending on the day where 1 = Sunday, 2 = monday etc. So the equation is basicaly saying add the difference between Friday (Day 6) and the tested dates day!
If you might be testing Saturday then use an Iif statement: IIf(Weekday(TestingDate)=7,TestingDate+6,TestingDate + 6 - Weekday(TestingDate))
HTH