Avoid warnings in unit tests.

[kalvdans]

This change removes some warnings that appear when running the unit tests:

luigi/luigi/file.py:26: DeprecationWarning: luigi.file module has been moved to luigi.local_target
luigi/luigi/parameter.py:259: UserWarning: Parameter 1 is not of type string.
luigi/luigi/parameter.py:259: UserWarning: Parameter 10 is not of type string.

It also allows the luigi binary to set self.complete_after to integer type, when using the --complete-after command-line flag.

[mention-bot]

@kalvdans, thanks for your PR! By analyzing the history of the files in this pull request, we identified @erikbern, @davemt and @Tarrasch to be potential reviewers.

[Tarrasch]

Thanks! :)