Ensure tests that create background=true indexes don't run at the same time

MongoDB only permits creating a single index on a database in the background at a time. If multiple background index builds are attempted, then a BackgroundOperationInProgressForDatabase error response is returned until the first completes. In order to avoid intermittent failures, we should either (a) add tests that create a background index on the "test" database to serialTestsArr, or (b) use a unique database (i.e. test-specific) in jsCore tests that create a background index.