explain_sample.js in core suite must be banned from running in parallel suites for non-WT storage engines

The test expects WT's storage-optimized sample implementation. It is tagged with requires_wiredtiger. However, this is not sufficient, since the parallel suite (basic.js and basicPlus.js) do not consult these tags when selecting jsCore test files. As a result, the parallel suite is failing for the ephemeralForTest build variant.