collation.js should use test-specific database

As part of collation testing, core/collation.js will convert a non-capped collection to one that is capped via "convertToCapped" command, confirming that the default collation of the original collection is preserved.

As part of the capped conversion, mongod will confirm that there are no background operations occurring via assertNoBgOpInProgForDb() call, asserting if there is a conflict.

collation.js uses the "test" database for its operations. As we run collation.js as part of the parallel suite, this raises the possibility that the assertNoBgOpInProgForDb() will fail due active background index build on the same database by another test. We should consider changing collation.js to use a test-specific database to avoid potential conflict.