Improve test-coverage of sessionctx/variable/variable.go

## Enhancement

The sessionctx/variable package still has quite low test coverage (~45%). A lot of this is because the individual sysvars (sysvar.go) don't always have tests. But there is no reason why the API in variable.go (currently at 91.9%) can't be at 100%. It is used by a lot of areas of the server, and undefined behaviors have a high risk of breaking upgrade/downgrade.