Allow for customizing default timeout

[swift-student]

Right now for confirmationOfCall we default the timeLimit to infinity (well, essentially infinity, Task.sleep doesn't handle anything over about Int32.max). This aligns with Apple's defaults for test timeouts like in XCTestExpectation:

fulfillment(of expectations: [XCTestExpectation], timeout seconds: TimeInterval = .infinity

I think this makes sense, as we don't want to set some arbitrary short timeout, but we should add the ability to change this default so that each consumer of TestDRS can decide on their own strategy without needing to specify timeouts on each use of confirmationOfCall.