Configure an IndexName for Oracle FTS tests

An IndexName became required with 8aad388 despite rt-fulltext-indexer's
fallback of "rt_fts_index", which is the default name created by
rt-setup-fulltext-index.  The test file failed because the indexer
didn't update the index due to FTS being turned off by the config check.
Avoid a warning; FIELD may not be passed to _SQLLimit

This warning crops up when an Oracle FTS query is run.  The _SQLLimit
provides a FUNCTION but no FIELD.