Replace usage of ConfigFlags.Input.InitialTimeStamp with ConfigFlags.Input.TimeStamp
A large number of tests were adding a new Input.InitialTimeStamp
flag and using that in the configuration
of the McEventSelector
replaced this with usage of the existing Input.TimeStamp
flag for consistency.
This should fix the unit test failures that appeared as a result of !45619 (merged) being merged.
FYI @wlampl