tests: Add tailable cursor functional test.
Add a new test case to check whether we handle tailable cursors well.
tests: Clean up the GridFS test area too.
In order to be able to easily clean up the GridFS test area, use a
name derived from the collection, and store that.
Then drop the .files and .chunks collection in test_cleanup.
tests: Add a cleanup program.
The cleanup program will be run after the testsuite completed, and
will drop the test collection, if networked tests are enabled.