MDL-47649 csvlib: must not leave files open

I added a destructor to csv_import_reader ensure the file is closed.

I also ensured that the gradeimport_csv unit tests did not hold onto a
reference to the csv_import_reader object while the phpunit framework
tries to tidy up moodledata.

Finally, to stop CiBoT complaining, I fixed up the PHP4-style
declarations in the CSV import class.